Galatians 2:20 (NLT)
20 My old self has been crucified with Christ. It is no longer I who live, but Christ lives in me. So I live in this earthly body by trusting in the Son of God, who loved me and gave himself for me.
Here the apostle describes his own mysterious life of being a believer in Christ. The old self of sin was crucified with Christ. But he lives and is comforted with the grace that came from Christ. While living in the earthly body still subject to sin he does so trusting Christ for guidance for he knows Christ loves him and gave His live for him. This is the Christian life we can all live.
[Theologian Alister McGrath outlines the following three stages of receiving what Christ did for us on the cross:
[First], I may believe that God is promising me forgiveness of sins; [second], I may trust that promise; but [third] unless I respond to that promise, I shall not obtain forgiveness. The first two stages of faith prepare the way for the third, without it they are incomplete.
Then he illustrates these three stages with the following true story:
Consider a bottle of penicillin, the famous antibiotic identified by Alexander Fleming, and first produced for clinical use in [Great Britain]. The drug was responsible for saving the lives of countless individuals who would otherwise have died from various forms of blood poisoning. Think of the three stages of faith like this. I may accept that the bottle exists. I may trust in its ability to cure blood poisoning. But nothing will change unless I receive the drug which it contains. I must allow it to destroy the bacteria which are slowly killing me. Otherwise, I have not benefited from my faith in it.
It is the third element of faith which is of vital importance in making sense of the cross. Just as faith links a bottle of penicillin to the cure of blood poisoning, so faith forges a link between the cross and resurrection of Jesus Christ and ourselves. Faith unites us with the risen Christ, and makes available to us everything he gained through his obedience and resurrection. [Alister E. McGrath, What Was God Doing on the Cross (Zondervan, 1992), pp. 99-100]]
The story above tells us that we can believe, but until we surrender to our own beliefs we have not followed through in faith. It is one thing to know a plane flies, but it another thing to trust sitting on the plane and hand control over to someone else to fly you up into the sky.