Unbelief is that state of mind which refuses to learn; it means the closed mind. To him “who has ears to hear,” Christ says, “I am the Christ.” To refuse to listen means that we are obstinate; it is sinful disobedience. But it is only to the believing soul that Christ says who He is. He Himself discards His incognito to faith, in faith. This is why, as the Christ, He remains concealed within history. Where He discloses Himself history disappears, and the Kingdom of God has begun. And when He unveils Himself He is no longer an historical personality, but the Son of God, who is from everlasting to everlasting.
This is the meaning of the distinction between “in the flesh” and “after the flesh.” For the historical student, for the historian and biographer, He remains the Rabbi Jesus of Nazareth, or the religious genius. This is the “Christ after the flesh,” and to know Christ in this way is to know Him “after the flesh,” even when such knowledge consists in the most profound and penetrating understanding of the personality of Jesus. But to know the “Christ come in the flesh” is to “know Him according to the Spirit”; this is the knowledge of faith, the knowledge of the Eternal Son of God as the “Word made flesh.”
(Emil Brunner, The Mediator, p.346)