John Gill Commentary Psalms 44:21

"Will not God search this out? For he knoweth the secrets of the heart." — Psalms 44:21 (ASV)

Shall not God search this out ?
&c.] Undoubtedly he would, was it so, and expose it, and punish for it; as he will the Balaamites and children of Jezebel, (Revelation 2:18–23) ; this seems to be an appeal to God for the truth of all that the church had said concerning her steadfastness and integrity under the most trying exercises;

for he knows the secrets of the heart ;
whether the heart is turned back, or there is any inclination to apostatize from God, or his name is forgotten in it; as well as whether in fact the hand has been stretched out, or prayer made to a strange god, (Jeremiah 17:9Jeremiah 17:10) .