“And that ye put on the new man, which after God is created in righteousness and true holiness.” Ephesians 4:24 (KJV)
This is the way it used to be 2,000 years ago, when people were converted to Christianity. They would be taught about their new faith for a while, and then they’d be baptized. They would go to a river, if they could, or a pond or lake if a river wasn’t handy. The person to be baptized would take off his old clothes and enter the water naked. After he had been baptized, then he’d come out of the water and put on new clothes; often he would be given a white robe to put on as a symbol of his new life in Christ.
Our baptisms didn’t happen that way, but I think we can still see the point. Becoming a Christian means putting off the old way of life and putting on a new way.
By the “new man” is meant the new nature, the new creature, which is actuated by a new principle, even regenerating grace, enabling a man to lead a new life, that life of righteousness and holiness which Christianity requires. This new man is created, or produced out of confusion and emptiness, by God’s almighty power, whose workmanship it is, truly excellent and beautiful.
We are said to put on this new man when, in the use of all God’s appointed means, we are endeavoring after His divine nature, this new creature. This is the more general exhortation to purity and holiness of heart and life. – Matthew Henry
The whole matter of putting on the new man is in essence the application of truth to ourselves. It is the most important thing that one can ever discover in the Christian life. The real secret of Christian living is to discover the art of talking to yourself. We must talk to ourselves, we must preach to ourselves, and we must take truth and apply it to ourselves, and keep on doing so. That is the putting on of the new man. We have to hammer away at ourselves until we have really convinced ourselves. In other words, this is not something that you wait for passively. If you wait until you feel like the new man it will probably happen. – D. Martyn Lloyd-Jones