Genesis 17:5 (NKJV)
Sometimes, for different reasons, people change their name. The most common example is when a woman gets married, her last name is changed. But even if this is not the case, when a woman gets married, she is now a wife and when a man gets married, he is a husband. When someone has their name changed for whatever reason they need to use that name for their identification and not their former name, otherwise there could be problems. Calling yourself by the right name is important so that you can be identified correctly.
In Genesis we see the story about Abraham. Originally his name was Abram, but when he got into a covenant relationship with God, God changed Abram's name to Abraham. There are several things that could be said about that, but the main thing is that there was a name change and for Abraham to correctly identify himself, he needed to refer to himself the way that God did. In the same way that God changed Abram's name to Abraham, when we come into a relationship with God through Jesus, what we call ourselves should change.
Once, we were a stranger to God, but now we are fellow citizens with the saints (Ephesians 2:9). We are now a new creation and the righteousness of God in Christ (2 Corinthians 5:17, 21), we are more than a conqueror in Christ (Romans 8:37), and the greater one, the Holy Spirit, lives in us (1 John 4:4). As a child of God and joint-heir with Jesus Christ (Romans 8:16-17) we need to know what to call ourselves. Don't refer to yourself the way you have before, call yourself who you are in Christ; we need to say about ourselves what God has said.