When the Bible speaks about our creation, it teaches us that we were united to Adam in creation. He was the head of the whole human race. He was our representative before God. He was like our priest before God, our king before God, and our prophet before God. When he fell, in him we all fell. This is what Paul teaches in Romans 5:12–21.
When Christ comes, He unites us to Himself, first by taking on our flesh so that we can speak about an incarnational union—He bonds Himself to us by taking our humanity. Then Christ does everything that is needed for our salvation, sends His Holy Spirit, and bonds us to Himself by the Holy Spirit so that we begin to taste and enjoy everything that He has done for us. Those are the basic elements of union with Christ.
Our union with Christ is a central theme in the New Testament. The word “Christian” is used only three times in the New Testament. New Testament believers seem to have thought of themselves fundamentally as people who had been given a new identity such that they were no longer united to Adam. Rather, they had been brought into a new humanity in Jesus Christ. All the riches of His grace were given to them to transform them into His likeness and ultimately to lead to the fruition of that union when we see Him face to face.