John Gill Commentary 1 Samuel 23:9

"And David knew that Saul was devising mischief against him; and he said to Abiathar the priest, Bring hither the ephod." — 1 Samuel 23:9 (ASV)

And. David knew that Saul secretly practised mischief against
him
That is, plotted and contrived it, formed schemes in order to do him mischief, giving out one thing, and designing another; so he pretended war against the Philistines, but his intention was to come against Keilah, and take David there:

and he said to Abiathar the priest, bring here the ephod ;
not for David to put on, but for the priest himself, that being clothed with it, and the Urim and Thummim in it, he might inquire for him of the Lord.