Taken from Matthew Henry's Commentary on  1 Samuel 19:18-24  

1. When the messengers came into the congregation where David was among the prophets the Spirit of God came upon them, and they prophesied, that is, they joined with the rest in praising God. Instead of seizing David, they themselves were seized. And thus,

 (1.) God secured David; for either they were put into such an ecstasy by the spirit of prophecy that they could not think of any thing else, and so forgot their errand and never minded David, or they were by it put, for the present, into so good a frame that they could not entertain the thought of doing so bad a thing. 

(2.) He put an honour upon the sons of the prophets and the communion of saints, and showed how he can, when he pleases, strike an awe upon the worst of men, by the tokens of his presence in the assemblies of the faithful, and force them to acknowledge that God is with them of a truth, 1 Cor 14:24,25. See also the benefit of religious societies, and what good impressions may be made by them on minds that seemed unapt to receive such impressions. And where may the influences of the Spirit be expected but in the congregations of the saints? 

(3.) He magnified his power over the spirits of men. He that made the heart and tongue can manage both to serve his own purposes. Balaam prophesied the happiness of Israel, whom he would have cursed; and some of the Jewish writers think these messengers prophesied the advancement of David to the throne of Israel. 

 2. Saul himself was likewise seized with the spirit of prophecy before he came to the place. One would have thought that so bad a man as he was in no danger of being turned into a prophet; yet, when God will take this way of protecting David, even Saul had no sooner come (as bishop Hall expresses it) within smell of the smoke of Naioth but he prophesies, as his messengers did, v. 23. He stripped off his royal robe and warlike habiliments, because they were either too fine or too heavy for this service, and fell into a trance as it should seem, or into a rapture, which continued all that day and night. 

The saints at Damascus were delivered from the range of the New-Testament Saul by a change wrought on his spirit, but of another nature from this. This was only amazing, but that sanctifying—this for a day, that for ever. Note, Many have great gifts and yet no grace, prophesy in Christ's name and yet are disowned by him, Matt 7:22,23. Now the proverb recurs, Is Saul among the prophets? Then it was different from what it had been, but now contrary. He is rejected of God, and actuated by an evil spirit, and yet among the prophets.