If you are a child of God, can you stop being his child when you do wrong?  Absolutely not.  You are kept by his grace and mercy through the power of the crucifixion and resurrection.  The big question is whether or not you are a child of God in the first place.  Have you totally and completely given all that you are to him?  If so, God said that no one could remove you from his hand.

One of the ways you can know you are His is by your changed life.  You aren't the same.  You want to please God and not the world.  Your paths are directed by His laws and desires.  You want His will in your life.

Which brings me to some very  controversial verses in Hebrews 6:4-6 "There is no use trying to bring you back to the Lord again if you have once understood the Good News and tasted for yourself the good things of heaven and shared in the Holy Spirit, and know how good the Word of God is, and felt the mighty powers of the world to come, and then have turned against God.  You cannot bring yourself to repent again if you have nailed the Son of God to the cross again by rejecting him…"

You must respond to the voice of God when it comes to you.  You have no assurance that he will call you again.  And once you have had that opportunity, you have no assurance the Holy Spirit will deal with you later.  I told you that the most horrible words in the Bible are:  "…and God gave them up…"

Perhaps another meaning of those verses is that the writer wants us to understand that you can't nail Jesus to a cross more than once.  He died once for sin.  Otherwise we might as well go back to the Jewish model and sacrifice lambs, calves, doves…for sin.  Which was only a prototype of the Lamb of God.  If Christ can't save us, what was the point of his death.

You may break your fellowship with God, but you can never break the relationship.  Once you are his child, you will always be his child. You have been adopted into the family of God.  Joint heirs with Christ.