Let your mind be spiritual renewed and put on the new man, created by God in his likeness. Ephesians 4:23
Paul advised the Ephesians in the Lord not to imitate the pagans who live in aimless kind of life. Their understanding was in darkness and they remained in ignorance because of their blind conscience, very far from the life of God.
We did not learn this from Christ. We were taught according to the truth that is in him, Jesus. We must give up our former way of living, whose deceitful desires bring self-destruction and put on the new man, created by God in his likeness, in holiness.
The old man and the new man both exist in society today and in each of us to a certain point. On one hand, we can be carried away by greed to the point of silencing our conscience and hardening our heart in a mortal way; and on the other hand, Christ wants to transform and renew us through his Spirit. We all tend to be one of these two persons, the old, ruined man without hope and a slave to his egotism; or a person transfigured by love.
Old man, new man corresponds to other of St Paul’s formulation : To live according to the flesh, or to the Spirit. Those walking to the flesh tend towards what belongs to the flesh; those led by the Spirit, to what is Spirit (Romans 8:5); to be children of darkness, or of light; to be slaves of sin, or to be free in Christ. These are the two alternative in life; the transition from one to other is what makes a Christian : A new man who is born again, renewed in Spirit.
Those who hope in the Lord will renew their strength.