[circa 1056 BC] Verse 1: And (1 Sam. 29:1) it came to pass in those days, that the Philistines gathered their armies together for warfare, to fight with Israel. And Achish said unto David, Know thou assuredly, that thou shalt go out with me to battle, thou and thy men.
[The Philistines assembled, etc.] Because they saw that David was removed, and that Samuel was dead. This came to pass by the admirable providence of God. For, Saul had now filled up the measure of his sins, and it was time that he should pay the penalty (Martyr).
[וַיִּקְבְּצ֙וּ פְלִשְׁתִּ֤ים אֶת־מַֽחֲנֵיהֶם֙ לַצָּבָ֔א] And they gathered their camps into (or unto [Jonathan]) the army (Montanus), or unto the fight (Pagnine), or to military service (Junius and Tremellius), that is, their troops to battle (Mariana). Their military troops, or to military service (Vatablus); their squadrons, so that they might compose the army (Munster); they were assembling their squadrons, and were enlisting the army (Tigurinus).
The Philistines were encouraged by Samuel’s death, and Saul’s degeneration, and David’s presence with Achish.
[With me thou shalt go out, etc.] Here David is held, caught in a snare; lest either he should do harm to the republic, or be ungrateful towards the one to whom he was owing his safety. He appears hardly able to avoid sin. He was put into these straits, because he had taken counsel from the flesh, rather than from God (Martyr).
Thou shalt go out with me to battle: this he saith, partly to try his sincerity; and partly in confidence of David’s fidelity.
Verse 2: And David said to Achish, Surely thou shalt know what thy servant can do. And Achish said to David, Therefore will I make thee keeper of mine head for ever.
[Now thou shalt know what thy servant is going to do, לָכֵן֙ אַתָּ֣ה תֵדַ֔ע וגו״] Surely (hence, or thence [Munster, Arabic]; thus [Jonathan], thus now [Septuagint], therefore [Tigurinus, Castalio]) thou shalt know, etc. (Pagnine, Montanus, etc.). In this manner thou shalt prove (Syriac, Junius and Tremellius); or on that account, etc. (Vatablus, Piscator), that is, because thou considerest me worthy of this honor, and hast so much confidence in me. He promises this ἑκὼν ἀέκοντι γε θυμῷ, willingly, but also reluctantly, as formerly Themistocles did with the Persians (Grotius). As the words sound, thus Achish understood them (Menochius). He responds ambiguously, which does not appear worthy of the candor of David (Martyr). David wanted to be understood of his industry on behalf of the Philistines in the war: but he was understanding it concerning his industry in betraying the army of the king (Piscator). David deliberately affirms nothing certain to King Achish, because he had not yet decided what he was going to do in the battle, but had determined to relinquish the whole matter to the divine will (of which he was going to inquire) (Tirinus, thus Menochius, Tostatus, Burgos and Salian in Tirinus). But God in His providence consulted both the reputation of David and his conscience (Tirinus). [At this point, Interpreters ask what David was going to do.] Response 1: He was going to fight for Achish against Saul. 1. Because he was bound to Achish on account of the benefits received. 2. It was able to happen, that the war of Achish against Saul was just, perhaps on account of certain agreements violated by Saul. 3. David had a right in the kingdom of Israel, which he was able to acquire by virtue of arms, and by the help of the Philistines (Lyra). But David was not yet King (as it is evident from this, that he was always acknowledging Saul to be King and his lord), neither was it lawful for him to claim his right by arms, or to fight against his own people (Martyr). Moreover, he was so far from desiring the destruction of Saul, that he killed his slayer (Cajetan in Sanchez). Response 2: David wished to betray Achish (Cajetan in Sanchez). They judge this to be probable from this, that he, having been declared King, immediately undertook war against the Philistines, although he was now looking to his own life by their protection. But they are deceived; for the Philistines were not incited by David, but they made war on him unprovoked (Martyr). Response 3: David wished to fight neither against the Philistines, nor against his own people; but he awaited some divine occasion, that he might be compelled to stand neither by the former, nor by the latter (Martyr).
David speaks ambiguously, as he did before, that Achish might understand him, as he did, of his acting for him against the Israelites; whereas he meant it of his acting for the Israelites against the Philistines, to which he was obliged both by God’s express command, and by his indelible and manifold obligations to God, and to God’s people, and by his own manifest interest. Though it is likely he would have managed his affairs with all possible regard and care of Achish’s person, to whom alone, upon the matter, David stood obliged, and not to the rest of the Philistines, who had an ill opinion of him, as we shall see.
[And I place thee as the keeper, etc., לָכֵן וגו״] For this reason: that is, because I see thee thus ready to serve me (Vatablus).
Therefore; for that valour which I doubt not thou wilt show on my behalf. Of mine head, that is, of my body and life, the captain of my lifeguard.
 Hebrew: וַֽיְהִי֙ בַּיָּמִ֣ים הָהֵ֔ם וַיִּקְבְּצ֙וּ פְלִשְׁתִּ֤ים אֶת־מַֽחֲנֵיהֶם֙ לַצָּבָ֔א לְהִלָּחֵ֖ם בְּיִשְׂרָאֵ֑ל וַיֹּ֤אמֶר אָכִישׁ֙ אֶל־דָּוִ֔ד יָדֹ֣עַ תֵּדַ֗ע כִּ֤י אִתִּי֙ תֵּצֵ֣א בַֽמַּחֲנֶ֔ה אַתָּ֖ה וַאֲנָשֶֽׁיךָ׃ צָבָא can signify either army, or war/battle.  Hebrew: וַיֹּ֤אמֶר דָּוִד֙ אֶל־אָכִ֔ישׁ לָכֵן֙ אַתָּ֣ה תֵדַ֔ע אֵ֥ת אֲשֶׁר־יַעֲשֶׂ֖ה עַבְדֶּ֑ךָ וַיֹּ֤אמֶר אָכִישׁ֙ אֶל־דָּוִ֔ד לָכֵ֗ן שֹׁמֵ֧ר לְרֹאשִׁ֛י אֲשִֽׂימְךָ֖ כָּל־הַיָּמִֽים׃ לָכֵן can signify according to such conditions, that being so, or therefore.  Paulus of Burgos, or Pablo de Santa Maria (c. 1351-1435), was a Spanish Rabbi, and a Talmudic and Rabbinic scholar. He converted to Roman Catholicism, took the degree of Doctor of Theology, and was made Archbishop of Burgos. His reputation as an exegete was secured by his Additiones to Lyra’s Postilla.  See 2 Samuel 1.