“Paul draws a contrast between the union of the unbeliever with Adam and the union of the believer with Christ. This is the great argument in Romans 5, which is repeated in 1 Corinthians 15:22, 49. In Romans 5 the whole argument is that death passed on to all people because of Adam. Why? Because of their relationship to Adam; that is the whole doctrine of original sin. We are all condemned in Adam because of Adam’s sin. He was our representative, he was our federal head; and not only that, we are bound to him, we were in the loins of Adam when he fell. In Adam all died. In Christ all shall be made alive again. That is it. The relationship of the believer to Christ is the same sort of union and relationship as that old relationship of the whole of Adam’s posterity to Adam. We are all born in Adam, and we are related, we are joined in that way. Yes, but being born again, we are in the same sort of relationship to Christ.
Regeneration and union must never be separated. You cannot be born again without being in Christ; you are born again because you are in Christ. The moment you are in Him you are born again, and you cannot regard your regeneration as something separate and think that union is something you will eventually arrive at. Not at all! Regeneration and union must always be considered together and at the same time because the one depends on the other and leads to the other; they are mutually self-supporting.
There is nothing that so strengthens my faith and fills me with a longing to be pure as He is pure and to live even as He did in this world as the realization of what I am and who I am because I am a Christian. I am a child of God, and I am in Christ.”
D. Martyn Lloyd-Jones. Walking with God Day by Day: 365 Daily Devotional Selections. Wheaton: Crossway, 2003. Appeared originally in God the Holy Spirit pp. 104-105.