But it was thou, a man mine equal - Margin, "a man according to my rank." Septuagint, ἰσόψυχε isopsuche, equal-souled, like-souled, "second self" (Thompson); Vulgate, "unanimus," of the same mind; Luther, "Geselle," companion. The Hebrew word used here - ערך ‛êrek - means properly a row or pile, as of the showbread piled one loaf on another, Exodus 40:23; then it would naturally mean one of the same row or pile; of the same rank or condition. The word also means price, estimation, or value, Job 28:13; Leviticus 5:15, Leviticus 5:18; Leviticus 6:6. Here the expression may mean a man "according to my estimation, value, or price;" that is, of the same value as myself (Gesenius, Lexicon); or more probably it means a man of my own rank; according to my condition; that is, a man whom I esteemed as my equal, or whom I regarded and treated as a friend.
My guide - The word used here properly denotes one who is familiar - a friend - from the verb אלף 'âlaph - to be associated with; to be familiar; to be accustomed to. The noun is frequently used to denote a military leader - the head of a tribe - a chieftain; and is, in this sense, several times employed in Genesis. 36 to denote the leaders or princes of the Edomites, where it is rendered duke. But here it seems to be used, not in the sense of a leader or a guide, but of a familiar friend.
And mine acquaintance - The word used here is derived from the verb to know - ידע yâda‛ - and the proper idea is that of "one well known" by us; that is, one who keeps no secrets from us, but who permits us to understand him thoroughly. The phrase "mine acquaintance" is a feeble expression, and does not convey the full force of the original, which denotes a more intimate friend than would be suggested by the word "acquaintance." It is language applied to one whom we thoroughly "know," and who "knows us;" and this exists only in the case of very intimate friends. All the expressions used in this verse would probably be applicable to Ahithophel, and to the intimacy between him and David.

But [it was] thou, a man mine (k) equal, my guide, and mine acquaintance.
(k) Who was not only joined to me in friendship and counsel in worldly matters, but also in religion.

But it was thou,.... The Targum is, "but thou Ahithophel"; of whom the words are literally to be understood, and so they are in the Talmud (u); and mystically and typically of Judas;
a man mine equal; "a man", and not a beast, nor a devil; but a man, from whom humanity, kindness, and tenderness might have been expected; though both Ahithophel and Judas acted the part of a devil; and the latter is expressly called one, John 6:70; "mine equal"; or like unto me; as the Targum. Ahithophel was not equal to David in dignity, as the king of Israel; nor in gifts, as the sweet psalmist of Israel; nor in grace as he; but as a man, a mortal dying man: kings and subjects are of the same blood, equally liable to death, and in the grave will be manifestly on a level: or rather the sense is, that he was in his esteem and affliction as himself; he was his friend that he loved as his own soul: so Judas could not be in every sense equal to Christ who is Jehovah's fellow, and thought it no robbery to be equal with God. Indeed as a man he was like unto him; a frail, mortal man, though not sinless as Christ. The word may be rendered "according to my appointment" (w), ordination, or constitution; Judas being a man appointed and ordained to be an apostle of Christ with the rest: or, "according to my esteem" (x); being had in great esteem and familiarity with Christ: or, "according to my order" (y), rank and class; being taken into his family, admitted to his table, where be sat down and ate with him, as if he was his equal;
my guide: or "governor" (z). Ahithophel was not governor over David; but was made a governor by him: he was one of his dukes or nobles, as the word is rendered in Genesis 36:15, was raised to great dignity by him; perhaps was chief minister of state: it is certain he was his counsellor, and his counsel was with him as the oracle of God, 1-Chronicles 27:33; he was his guide in civil affairs; he was directed by his advice, and it may be was president of his privy council. Judas was not only the guide of them to Christ who took him, Acts 1:16; but when the apostles were sent out two by two before the face of Christ, to preach where he himself should come, Judas was sent also, Mark 6:7;
and mine acquaintance: one well known to him, as Ahithophel was to David, and Judas to Christ, his friend and companion, in whom he confided, and who ate of his bread; and all these characters are so many aggravations of his treachery and wickedness.
(u) T. Bab. Sanhedrin, fol. 106. 2. Pirke Abot, c. 6. s. 3. (w) "Secundum dispositionem, sc. ordinationem et constitutionem meam", Calvinus in Michaelis. (x) "Juxta estimationem meam", Vatablus, Gejerus, Michaelis; "qui par mihi estimatus est", Piscator. (y) "Secundum ordinem meum", Mollerus. (z) "dux meus", Pagninus, Tigurine version, Junius & Tremellius, Piscator; "princeps meus", Cocceius.

guide--literally, "friend" (Proverbs 16:28; Proverbs 17:9).
acquaintance--in Hebrew, a yet more intimate associate.

Equal - Not in power, but in reputation, for wisdom, and influence upon my people. Guide - Whose counsel I highly prized, and constantly followed. All which agrees to Achitophel.