Adam Clarke on Acts 15:8

And God which knoweth the hearts - Ο καρδιογνωϚης Θεος. We had this epithet of the Divine Being once before; see Act 1:24, and the note there: it occurs no where else in the New Testament.

Bare them witness - Considered them as proper or fit to receive the Gospel of Christ. It is properly remarked by learned men, that μαρτυρειν τινι, to bear witness to any person, signifies to approve, to testify in behalf of. Here it signifies that, as God evidently sent the Gospel to the Gentiles, and, by the preaching of it, conveyed the Holy Spirit to them who believed, and as he can make no improper judgment of any who knows all hearts and their secrets, therefore what he had done was right: he saw that it was time for them to receive the Gospel; and he saw that they might be safely trusted with this heavenly deposit; and the experience of eighteen hundred years has justified the conduct of God.