There are two aspects to the spiritual life which comes after you have died to the old man (your prior life). One is called sanctification and the other is called justification.
We cannot sanctify ourselves. That is the work of the Holy Spirit in our spiritual life. When we first come to Jesus, though, He sanctifies us in that moment and, in that moment, we are saved. In that moment, there is nothing more to be done for our salvation. But, life goes on. Likely, in the very next moment we may sin. Why does that happen? Though Jesus had sanctified us, we were not yet perfected. Thus, the Holy Spirit continues that work of sanctification throughout our lives. The Holy Spirit's influence is what keeps us coming back to Christ for sanctification.
In the mean time, what are we to do? Just as faith brought us to Christ in the first place, it is by faith that we begin to demonstrate our willingness to keep all of God's Law. This is justification. Justification is not about being perfect, it is only about demonstrating a willingness, by faith, to keep all of God's Law at the same time that the Holy Spirit continues to sanctify us to the point where Jesus Christ's requirement of us (Mat. 5:48) becomes a reality -- we become perfect.
Demonstrating that faith justifies us *in* our faith. But a live, viable faith, results in a desire to keep all of God's Law -- which further justifies us. As we become perfected (sanctified), we become better able to keep all of God's Law perfectly. Each time we keep one of God's Laws, we are justified by that action. "Was not Abraham our father justified by works, when he had offered Isaac his son upon the altar?" (James 2:21) Why was he justfied? Because he obeyed out of a love for God -- not out of a sense of duty. If we aren't willing to obey (justified), then it proves we are not really saved (sanctified), either.
God bless.