John Gill Commentary 1 Samuel 18

And it came to pass, when he had made an end of speaking unto
Saul
In answer to his questions about his descent and family, and doubtless more things were talked of than are recorded:

that the soul of Jonathan was knit with the soul of David :
he won his heart, made a conquest of his affections, these went out towards him, and cleaved unto him; such were the comeliness of his person, his graceful mien and deportment, his freedom and fluency of expression, his courage and undauntedness, joined with prudence, modesty, and integrity, that they strongly attached him to him:

and Jonathan loved him as his own soul ;
not only according to the excellency of David's soul, and the greatness of it, as that deserved respect and love, as Abarbinel suggests, but he loved him as he loved himself. There was a similarity in their persons, in their age, in the dispositions of their minds, in their wisdom, courage, modesty, faithfulness, and openness of soul, that attracted them to each other, that they became as another self; as one soul, as Aristotle speaks ^F18 of true friends: instances of very cordial friendship are given by Plutarch ^F19 , as in Theseus and Pirithous, Achilles and Patroclus, Orestes and Pylades, Pythias and Damon, Epaminondas and Pelopidas; but none equal to this.

FOOTNOTES:

• F18: Ethic. l. 9. c. 4, 9. So Porphyr. de Vita Pythagor.
• F19: Apud Patrick in loc.

And Saul took him that day
Not only into his favor, and into his service, but into his court; even on that very day he slew the Philistine, or however as soon as it could be done:

and would let him go no more home to his father's house ;
as he used to do before; when he only served as a musician to him, then he was only at court when Saul was in a melancholy disposition, and wanted him, and so was going and returning, and in the intervals kept his father's sheep, (1 Samuel 17:15) ; but now he would not suffer him to attend such business any longer, since he was not only to become a courtier, and be made a prince or noble, but to marry his daughter, according to the declaration he had made, with respect to any man that should kill Goliath.

Then Jonathan and David made a covenant
A covenant of friendship; entered into a solemn agreement to keep up and maintain a cordial respect to each other, and to support each other's interest both in life and after death, whoever was the survivor; and in consequence of this David had a friend at court, when Saul fell out with him, and who pleaded his cause, and discovered his father's plots, and was the means of preserving David's life:

because he loved him as his own soul ;
so that this covenant was not founded in mere words, but in sincere and cordial affection, and was lasting and inviolable.

And Jonathan stripped himself of the robe that [was] upon him ,
&c.] As a token of his hearty love and true friendship, and that David might appear at court not in the habit of a shepherd, but in that of a prince:

and gave it to David, and his garments ;
his other garments besides his robe, and so clothed him from tip to toe, and which fitted him; for as there was a similarity in their souls, and the disposition of them, so in the make and hulk of their bodies, and in the stature of them:

even to his sword, and to his bow, and to his girdle ;
these he gave him to accoutre himself with, that he might appear as a soldier, as well as like a prince, and as another Jonathan, or rather the same; that they might seem as one, as alike in body, so in garb and habit.

And David went out whithersoever Saul sent him
About any business whatsoever, especially about martial affairs, for which he was abundantly qualified:

[and] behaved himself wisely ;
in the management of them, using great prudence and discretion, and so failed not of success, and of recommending himself; the Targum renders it "prospering"; he was prosperous and successful in whatsoever he engaged, for the Lord was with him, and blessed him:

and Saul set him over the men of war ;
that is, of some of them, gave him the command of a troop; for Abner was captain or general of the army, and continued so:

and he was accepted in the sight of all the people ;
of all the people in the land in general, of all that knew or heard of him; being looked upon as a wise, valiant, and successful commander, and which gained him the esteem and affection of the people:

and also in the sight of Saul's servants ;
which was very much, and a rare thing, for servants are too apt to envy such as are rising in their credit and reputation; though this must not be understood of all, without exception; but of the generality of them; nor is the word "all" used of them, as is of the people; for some of them took the part of Saul afterwards against David, and were secretly his enemies, see (1 Samuel 18:22–26) .