You become a Christian by believing in the Lord Jesus (Rom. 10:8-10). According to 1 Peter 2:2, when you first accept Jesus, you're like a baby. You can be fully mature in your body and mind, but still be a baby concerning spiritual things.
It's possible to be a Christian for 20 years or more and still be a spiritual baby. The only way to grow up spiritually is by feeding on the Word of God.
"Like newborn babies, crave pure spiritual milk, so that by it you may grow up in your salvation" (1 Peter 2:2).
A Christian that is a spiritual baby will stumble into sin over and over again. Remember we'll never be perfect until Jesus gives us our glorified bodies, but we can come to a place where sin is not a life-style. We do this by feeding on the Word of God and thus growing up spiritually.
Let me conclude with a teaching from Jesus.
To the Jews who had believed Him, Jesus said, "If you hold to my teaching, you are really my disciples. Then you will know the truth, and the truth will set you free."
They answered Him, "We are Abraham's descendants and have never been slaves of anyone. How can you say that we shall be set free?"
Jesus replied, "I tell you the truth, everyone who sins is a slave to sin. Now a slave has no permanent place in the family, but a son belongs to it forever. So if the Son sets you free, you will be free indeed.
(John 8:31-36)
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