An imitation of Christ is possible only when we are first of all aware of the fact that we are not able to imitate him.
He is sinless; we are not. He offers the sacrifice of atoning death; we cannot. It is precisely the decisive act of obedience which effects our perfection which we cannot imitate. In Hebrews and in Paul the connection between our perfection and the perfection of the High Priest can be understood only as happening in faith in the ephapax [Gk: the 'once-for-all'-ness] of the high priestly act.--Oscar Cullmann, The Christology of the New Testament (rev. ed.; 1963), 100-101