John Gill Commentary 1 Samuel 23

Then they told David
Either the men of Keilah sent to him, being near them, or some well wishers of theirs, and of their country, acquainted him with their case:

saying, behold, the Philistines fight against Keilah ;
had laid siege to it, being a fortified place, (1 Samuel 23:7) ; it was a city in the tribe of Judah, on the borders of the Philistines; of which (See Gill on Joshua 15:44);

and they rob the threshing floors ;
took away the corn upon them, which they were threshing and winnowing, which were usually done without the city for the sake of wind, see (Judges 6:11) (Ruth 3:2Ruth 3:15) ; it was harvest time when the three mighty men came to David in the cave of Adullam, and so now it might be the time of threshing, harvest being over, see (1 Samuel 22:1) ; compared with (2 Samuel 23:13) .

Therefore David inquired of the Lord, saying, shall I go and
smite these Philistines ?
&c.] For though David was well disposed to serve his country, and was desirous of freeing them from their enemies the Philistines, he might have some doubts in his mind whether it would be right for him to engage with them now; partly because he could not act under a commission from his prince, Saul the king; and partly because he had such a small number of forces with him, that it might be hazardous for him to attack the armies of the Philistines with them, and attempt to raise the siege of Keilah; and therefore he thought it advisable, as doubtless it was, to inquire of the Lord what was his mind and will in this matter:

how and by what means he inquired it is not said, very probably it was by the prophet Gad, who was with him, (1 Samuel 22:5); for as for Abiathar, he was not yet come with the ephod, the Urim and Thummim, to inquire by them, (1 Samuel 23:6); though some think that is observed there to show in what way David did inquire, namely, by Urim and Thummim; and so Kimchi and Abarbinel understand it; and it is supposed that he came to David when he was about Keilah, and near unto it, and so before he came thither, and time enough for him to inquire by him whether he should go thither or not:

and the Lord said unto David, go, and smite the Philistines, and save
Keilah ;
which was not only giving him leave to go, and signifying it was his mind and will he should; but that he should be successful, and rout the Philistines, and raise the siege of Keilah, and save the city from falling into their hands.

And David's men said unto him, behold, we be afraid here in
Judah
Of Saul and his army falling upon them, and crushing them, though they were in the tribe of Judah, where they had many friends, and in the heart of that tribe:

how much more then if we come to Keilah ;
which, though in the same tribe, yet in the further parts of it, and on the borders of the Philistines: and there engage

against the armies of the Philistines ?
too numerous and powerful for them, and so by this means be driven out of their place of safety, the forest of Hareth, where they could hide themselves upon occasion; to be exposed not only to the Philistines, before them, on the edge of their country, from whence they could have re-enforcements easily, but to Saul and his army behind them; and so, being between two fires, would be in danger of being cut off.

Then David inquired of the Lord yet again
Not for his own sake, who firmly believed it was the will of God he should go and succeed, but for the sake of his men, and to remove the doubts and fears that hung on their minds:

and the Lord answered him, and said, arise, go down to Keilah ;
immediately, make no stay, nor hesitate about it, but go with all haste to the relief of the place:

for I will deliver the Philistines into thine hands ;
which is still more explicit, and is a promise not only of delivering Keilah out of the hands of the Philistines, but of delivering them into David's hands, and so of an entire: victory; and therefore none of David's men had anything to fear after such a declaration of the will of God.

So David and his men went to Keilah
Animated by a commission from God, and a promise of success by him:

and fought with the Philistines ;
encamped before Keilah:

and brought away their cattle ;
which they had brought with them for the support of their army; or having routed them, they pursued them into their own country, and brought off their cattle from thence:

and smote them with a great slaughter ;
killed great numbers of them, and put the rest to flight:

so David saved the inhabitants of Keilah ;
from falling into the hands of the Philistines, by timely raising the siege of the city.