4 The Jews made this inquiry of Christ, chiefly in order that, should He say, I am Christ, they might, in accordance with the only sense they attached to such a name, that He was of the seed of David, calumniate Him with aiming at the kingly power. There is more than this in His answer to them: they wished to calumniate Him with claiming to be the Son of David. He replied that He was the Son of God. And how? Listen: “Jesus answered them, I tell you, and you believe not: the works that I do in my Father's name, they bear witness of me: but you believe not; because you are not of my sheep.” You have already learned above (in Lecture XLV.) who the sheep are: be ye sheep. They are sheep through believing, sheep in following the Shepherd, sheep in not despising their Redeemer, sheep in entering by the door, sheep in going out and finding pasture, sheep in the enjoyment of eternal life. What did He mean, then, in saying to them, “You are not of my sheep”? That He saw them predestined to everlasting destruction, not won to eternal life by the price of His own blood.
Source: Tractates on the Gospel of John (New Advent)