By the same Grace that provided you with salvation, the Holy Spirit came down to LIVE in you.
You became a new creation, a new man Spiritually. The Bible says the Old has passed away (we can deny and rebuke the old sinful fleshly nature) and the NEW now lives in you (you can live by the Spirit).
But we have to be TRANSFORMED through the Spirit by the renewing of our mind to complete the process.
2 Corinthians 5:17 (NKJV)
"Therefore, if anyone is in Christ, he is a new creation; old things have passed away; behold, all things have become new."