Shall not God search this out? - That is, If this had been the case, it would be known to God. If, as a nation, we had been given to idolatry, or if our hearts had been secretly alienated from the true God, though there had been no open manifestation of apostasy, yet that could not have been concealed from him. The question here asked implies a solemn declaration on the part of the psalmist that this was not so; or that there was no such national apostasy from God, and no such prevalence of idolatry in the land as to account for what had occurred. The reason for the calamities which had come upon them, therefore, must be found in something else.
For he knoweth the secrets of the heart - What is in the heart: what is concealed from the world. If there were any such alienation from him in the hearts of the people, he would know it. The fact that God knows the heart, or that he understands all the secret thoughts, purposes, and motives of people, is one that is everywhere affirmed in the Scriptures. See 1 Chronicles 28:9; Romans 8:27; compare the notes at Revelation 2:23.