Paul, an apostle of Christ Jesus by the will of God...
Paul was a murderer. He deliberately hunted down, harassed and even killed people who disagreed with him. He did this with religious zeal, thinking he was doing God a favour. Just a side note: some of the hardest-hearted, cruel people are those with religious zeal thinking they are pleasing God or gods.
So Paul had a past. Yet here we find him referring to himself as an apostle of Christ Jesus. How dare he! He had caused so much pain and suffering. Doesn't he know he should walk around in shame and guilt for the rest of his life?
Paul couldn't change his past, but what he could do, after repentance before Jesus, was realize he was a new creation. He was not the same man that he was before. He might look the same, and his personality might be the same, but he was a new man. The past was gone. [Now, I believe in restitution as part of the repentance process, but that is a different topic.]
How about you and I? Do we have a past? Have we done things, said things, been someone who hurt others? In Christ Jesus, we become a new person. While we have to deal with the past and its consequences, that is not who we are anymore. Its guilt and shame can be left at the foot of the cross. Like Paul, we can embrace who we are in Christ and move forward. If you are a Christian, you do not have to let your past define your future. You are a new person. Paul moved from murderer to apostle. You can make the move too.