Psalm 18 is a unique chapter: it appears almost verbatim in the Book of II Samuel, Chapter 22. We must understand why this psalm is repeated twice, and why there are variations in words and letters between the two locations. Let us begin with the heading of the psalm:
לַמְנַצֵּחַ לְעֶבֶד יְהֹוָה לְדָוִד אֲשֶׁר דִּבֶּר לַיהֹוָה אֶת־דִּבְרֵי הַשִּׁירָה הַזֹּאת בְּיוֹם הִצִּיל־יְהֹוָה־אוֹתוֹ מִכַּף כׇּל־אֹיְבָיו וּמִיַּד שָׁאוּל׃ - For the conductor, by the servant of the Lord, by David, who spoke to the Lord the words of this song on the day the Lord delivered him from the hand of all his enemies and from the hand of Saul.
Rashi explains that David recited this psalm in his old age, after all his troubles had passed and he had been delivered from them. Ibn Ezra explains that he composed it when his men swore to him that he would no longer go out to battle with them so that he would not quench the lamp of Israel - for it was a great danger for the king of Israel to die in battle. Both interpretations point to a later period, in the days of David's old age.
Saul was presumably included among his enemies, so why single him out? Rashi explains that Saul was harder on him and persecuted him more relentlessly than all the others. David had many enemies, but there was one enemy who distressed him more than anyone else. This is a common phenomenon in Tanach, and the Malbim cites further examples of it, such as "Go view the land and Jericho" - yet is Jericho not part of the land? Rather, the primary place is mentioned separately. So too here: the primary pursuer is mentioned individually by name.
An interesting point: the Malbim did not write a commentary on this song in II Samuel 22, but left it for Tehillim. However, he provided his introduction specifically there. Thus, in Samuel there is an introduction without a commentary, and in Tehillim there is a commentary without an introduction, and the two complement each other.
There, the Malbim asks: if the psalm was already arranged in the Book of Psalms, why was it presented again at the end of the Book of Samuel? And why are there variations in words and letters (such as cheskat mayim versus chashrat mayim, and the like)? In his answer, he relies on the words of the Mahari"a - Rabbeinu Yitzchak Abarbanel, whose insights he frequently cites.
The Abarbanel explains differently from both Rashi and Ibn Ezra. David composed this song in his youth, while he was still in the midst of his troubles, crafting it as an all-encompassing song for every distress - so that whenever Hashem delivered him from trouble, he would have an established psalm ready to sing. This is the meaning of "on the day the Lord delivered him": on every single day that he was saved, he would recite this psalm. It was the song that accompanied him at all times.
The Malbim adds: the Book of Samuel does not record David's songs and praises. The book chronicles his life events, but his praises are not included there - except for this psalm, precisely because it was the comprehensive song he sang on every occasion.
And what about Tehillim? In his old age, King David collected all his psalms and compiled them together into a dedicated book, for the spiritual guidance of the solitary soul in contemplation. That is to say: what he had once sung for himself became a book that any person could recite over their own personal troubles, finding in it a remedy for all afflictions. This psalm encompasses everything a person undergoes throughout life, and contains wondrous segulot as a work composed with divine inspiration, as noted in the book Shimush Tehillim; therefore, he recorded it once again in the Book of Psalms.
This also explains the variations: in Tehillim, the psalm is intended for the recitation of the individual, which is why David made adjustments so that it would be suitable for every single person. The text in Shemuel is a transcription of the way David prayed for himself, whereas in Tehillim these are adaptations he added and modified so that it would fit the needs of each and every person.
And the Malbim concludes there: "The commentary on this precious psalm is written with me in the Book of Tehillot, for that is its proper place, and I shall not duplicate it here unnecessarily; 'to its habitation shall you seek, and there you shall come' (Devarim 12:5), if God will be with me" - meaning that at that time the commentary was still among his manuscripts and had not yet been published, which is why he conditioned it upon "if God will be with me."
Regarding the superscription, "For the conductor, by the servant of Hashem, by David," the Malbim explains that David is giving thanks for Hashem saving him from all his enemies in his many wars, and especially from the hand of Shaul, through whom he was in greater danger than all of them. And in this psalm, he explained two aspects of the Holy One, Blessed is He's guidance toward him.
One is the guidance in times of tranquility: when there was no distress and no fear of enemies, David thanks and blesses Hashem for the very fact of being in a tranquil period, a time without evil. The second is the guidance during times when he had many bitter troubles. And in this section, he describes two kinds of providence and salvation: natural providence - salvation through natural means, which David merited not on account of his deeds but by virtue of his prayer; and supernatural providence - open miracles transcending the laws of nature, which he merited on account of his good deeds. For the miraculous salvation he does not even need prayer, for it comes to him in the merit of his deeds, as Hashem is with him.
וַיֹּאמַר אֶרְחׇמְךָ יְהֹוָה חִזְקִי׃ - And he said: I love You, Hashem, my strength.
First, regarding the correct pronunciation: as the Radak writes, the letter chet here is vocalized like a chataf kamatz. Wherever the chet appears with a sheva, it is read not as "ch" but as "cha" - just as we say "chachmah," and not as some mistakenly recite in the blessing of Chonen HaDa'at, "ch'chmah, binah, vada'at." And so too here: "Erchamcha," in the pattern of "Ezkorcha" - for even though it is spelled without a vav, it is read not as "ezk'rcha" but as "ezkorcha."
Rashi explains: "Erchamcha" - I will love You, from the expression "and you shall love your neighbor as yourself" (Vayikra 19:18), which the Targum renders as "vetircham," and "rechimei" means those who love him. That is to say: I love You, Hashem, my strength.
The Malbim explains differently: "Erchamcha" - I cause You to be filled with mercy. The explanation is this: human compassion toward another is an emotional reaction - I look at his plight, become moved, my heart aches for him, and I feel mercy. Such emotional reaction does not apply to the Holy One, Blessed is He, Who is entirely unaffected by anything or anyone. Consequently, it is inapplicable to ascribe to Him, blessed be He, mercy or cruelty, or any emotional attributes and traits - all of these are strictly from the perspective of the recipients. We receive divine bounty from Hashem, and from our perspective this is called "Hashem had mercy on me." Yet it is inaccurate to say that Hashem became filled with mercy, for in doing so we would render Him, as it were, affected: as though I acted upon Him and He underwent change on my account. The Holy One, Blessed is He, undergoes no change from anyone. Therefore, "erchamcha" means: I am the recipient on whose account You bestow mercy - the mercy is the outflow of bounty that I receive, not an arousal of emotional compassion within Him, blessed be He, which certainly cannot apply to the Divine.
However, one must know that when speaking of the Sefirot, it is indeed appropriate to say that they are influenced. The Holy One, Blessed is He, in His Essence, is unaffected by anything, but regarding the Sefirot this does apply - as we are in the mode of "Ascribe strength to God" (Tehillim 68:35), meaning that through our good deeds we grant strength to the Heavenly Court, and this is not within Hashem, blessed be He, Himself, but within the Sefirot. When the righteous perform their service, this is known as the elevation of Mayin Nukvin (feminine waters), which awakens love Above, and through this brings down Mayin Dechurin (masculine waters), the bounty that Hashem bestows. Therefore, when discussing the Sefirot, it is meaningful to speak in terms of influence and arousal, causing harm or creating mercy. Thus, for example, the Thirteen Attributes of Mercy (Yud-Gimmel Mechilan DeRachamei) are rooted in the world of Atzilut, in Atik and Arich, exalted realms corresponding to the mochin (intellect). These are dimensions of mercy and judgment within the Sefirot, but not within His Essence, blessed be He.
יְהֹוָה סַלְעִי וּמְצוּדָתִי וּמְפַלְטִי אֵלִי צוּרִי אֶחֱסֶה־בּוֹ מָגִנִּי וְקֶרֶן־יִשְׁעִי מִשְׂגַּבִּי׃ - Hashem is my rock, my fortress, and my deliverer; my God, my rock in Whom I take refuge, my shield, the horn of my salvation, my stronghold.
Rashi explains "Hashem is my rock" - that You aided me at the Rock of Divisions (Sela HaMachlekos), when I was hemmed in by Shaul and his men, on the verge of capture, as it is said, "Shaul and his men were encircling David." There was a moment when Shaul was a hairsbreadth away from laying his hand on David: he pursued him around the rock, and seeing that it was difficult to seize him, he sent another detachment to come from the opposite side and encircle him. David reached the point where he thought: It is over; there is no longer any way of escape. Yet at the very last moment, messengers arrived and informed Shaul that the Pelishtim were raiding Israel. Shaul understood that as crucial as the pursuit of David was, saving Am Yisrael from the Pelishtim was far more vital, and he immediately left the area to go and rescue Israel. The site was called "Sela HaMachlekos" - because they divided there into two detachments (machlakos) to pursue David. And this is "Hashem is my rock": at that very rock, You saved me.
The Malbim explains here the different periods David went through, and for each period - the nature of the psalm and the form of prayer appropriate to it. First, he describes the period of tranquility, during which he faced no distress whatsoever. In that state, his songs encompass two themes: first, protection and defense from enemies, as Hashem shields him so that they cannot approach him; and second, thanksgiving for prevailing over his enemies and reigning over them.
Regarding defense, he opens: "Hashem is my rock, my fortress, and my deliverer" - three distinct dimensions. "My deliverer" (mefalti) - that Hashem protects and rescues him from the enemies. "My fortress" (metzudasi) - as the standard method of defense was to flee into a stronghold or fortress, and Hashem is the fortress to which one flees. "My rock" (sal'i) - for the fortress was built atop a high mountain, upon a crag of rock, and Hashem is the rock upon which the fortress stands. That is to say: You are the rock, and the fortress, and the One Who delivers me safely into that fortress. In simple terms - I have no one to rely upon other than You; You are my defense and salvation in every respect.
Corresponding to the second level, where he exercises mastery over his enemies, he says: "My God, my rock in Whom I take refuge." The name "Eli" ("my God") indicates a hashgachah (providence) uniquely tailored to him. The Malbim explains this principle in several places: "Elokim" refers to Him as the Overseer of the entire world, whereas "Elokai" refers to His individual, personal providence over me. Thus: "Eli tzuri" - there is special providence over me, and through its strength I also vanquish my enemies.
To defeat enemies, three elements are needed: a shield - when a person marches out to war, he requires a shield, which is "my shield" (magini); a horn with which to gore the enemy, which is "the horn of my salvation" (keren yish'i); and strength, power, and courage during battle, which is "my stronghold" (misgabi). All of this is spoken during the period when David enjoyed tranquility and was neither hunted nor in desperate need of rescue - for David waged wars even during times of national peace. In those periods, he offers thanks that Hashem delivers him from enemies: my stronghold, my fortress, and my deliverer; and furthermore - that Hashem assists him in fighting them and prevailing over them: my shield, the horn of my salvation, and my stronghold.
מְהֻלָּל אֶקְרָא יְהֹוָה וּמִן־אֹיְבַי אִוָּשֵׁעַ׃ - With praise I call upon Hashem, and I am saved from my enemies.
Rashi explains: Even before the salvation arrives, I praise Hashem, because I am completely confident that I will be saved from my enemies. Seemingly, praise should only be offered after the deliverance has occurred - but David is entirely assured of it even before it arrives.
The Malbim explains: When I am not in distress - for here we are dealing with David's periods of tranquility - I call upon Hashem with the title "mehulal" (the Praised One), for He is constantly praised. In such a reality, he does not pray or plead as one does in times of crisis. A person enduring affliction pleads; here, however, David does not speak in the language of petition, but in the language of praise. I praise Hashem, and I have no need to supplicate at all, for through the very power of that praise - "and I am saved from my enemies." This is an ideal state in which I am saved by Hashem as a matter of course, and what remains for me? To laud and praise the Creator.
Psalm 18 is David's comprehensive song, which he composed in his youth in the midst of his troubles in order to recite it on every day he was delivered, and which he later arranged in the Book of Tehillim with textual variations so that it would suit every person in their times of distress. The psalm presents two modes of divine conduct: the conduct in times of tranquility and the conduct in times of trouble, and within the latter, two kinds of providence - natural providence through the power of prayer, and miraculous providence through the power of deeds. At the opening of the psalm, David expresses his deep devotion - "I love You, Hashem, my strength" - and enumerates three aspects of protection (my rock, my fortress, my deliverer) and three aspects of victory (my shield, the horn of my salvation, my stronghold), concluding in his time of tranquility not with supplication, but with praise: "Praised, I call upon Hashem, and I am saved from my enemies."