Then because He said above “And the sheep hear his voice, and follow him,” lest any should say, “What then is this to those who believe not?” hear what He addeth, “And I know My sheep, and am known of Mine.” As Paul declared when he said, “God hath not rejected His people whom He foreknew” ( Rom. xi. 2 ); and Moses, “The Lord knew those that were His” ( 2 Tim. ii. 19; comp. Num. xvi. 5); “those,” He saith, “I mean, whom He foreknew.” Then that thou mayest not deem the measure of knowledge to be equal, hear how He setteth the matter right by adding, “I know My sheep, and am known of Mine.” But the knowledge is not equal. “Where is it equal?” In the case of the Father and Me, for there, "As the Father knoweth Me, even so know I the Father.” Had He not wished to prove this, why should He have added that expression? Because He often ranked Himself among the many, therefore, lest any one should deem that He knew as a man knoweth, He added, “As the Father knoweth Me, even so know I the Father.” “I know Him as exactly as He knoweth Me.” Wherefore He said, “No man knoweth the Son save the Father, nor the Father save the Son” (Luke x. 22), speaking of a distinct kind of knowledge, and such as no other can possess.
John Chrysostom, Homily 60 on John