"17 Now this I say and testify in the Lord, that you must no longer walk as the Gentiles do, in the futility of their minds. 18 They are darkened in their understanding, alienated from the life of God because of the ignorance that is in them, due to their hardness of heart. 19 They have become callous and have given themselves up to sensuality, greedy to practice every kind of impurity. 20 But that is not the way you learned Christ!— 21 assuming that you have heard about him and were taught in him, as the truth is in Jesus, 22 to put off your old self,[f] which belongs to your former manner of life and is corrupt through deceitful desires,23 and to be renewed in the spirit of your minds, 24 and to put on the new self, created after the likeness of God in true righteousness and holiness."
Ephesians 4:17-24
In understanding the ideas expressed by Paul in the phrases 'the old man' and 'the new man' it will help us to look at other similar statements in his writings - Romans 6:1ff; Romans 7:1-6; Romans 13:14; Galatians 2:20; Colossians 3:1ff, all of which are closely linked together in thought. The Galatian statement is notable, in the double paradox that it expresses, 'I live, yet not I, but Christ liveth in me' - this is the 'new man', just as the 'I' in 'I have been crucified with Christ' signifies the 'old man', being done to the death and abandoned. And the 'new' is a 'replacement' life, which is 'Christ in us', yet which is nevertheless a true life for us. Perhaps the most helpful illustration is that in Romans 7:1-6, in the 'marriage illustration' Paul uses. It is a somewhat complex one, and makes us ask the question, 'Who is married to whom, in this illustration that Paul uses?' Well, the 'woman' here has two marriages, the second of which is to Christ (4). The 'woman' represents the essential personality, or 'ego', that has through the Fall become 'married' to, or saddled with, this brute of a husband, 'the old man'. It is this, first, marriage that causes all the trouble in human experience. But this 'old man', the first husband, is put to death in the death of Christ, and the essential 'ego', or personality is set free to marry another, even Christ. This is what is referred to in Galatians 2:20, 'I have been crucified with Christ ...' a crucifixion in which the old man is destroyed, and a new marriage takes place, with Christ. This is why in the words which follow 'Nevertheless I live ...' there is no contradiction. Rather, a new union or marriage takes place, for our association with Christ relates both to His death (by which 'the old man' is crucified and destroyed) and to His resurrection (by which we taste and share - by our new marriage - in His risen life). And of this second marriage, God says, as He said at the beginning, 'Be fruitful and multiply'. There is to be fruit of this new union; and the fruit of the first marriage is to be eschewed and turned from. This is what Paul spells out here in 4:25ff.