If you have sincerely committed your life to Jesus Christ and are trusting Him alone for your salvation, then He has adopted you into His family -- and nothing can ever change that. You are now His child, and you'll remain His child forever!
Think of it this way. When children are born, they become members of a family -- and they'll always be part of that family as long as they live. In time, they may do something wrong or rebel against their parents -- and if they do, their fellowship with their family will be broken. But their relationship can't be broken; they'll still be part of that family.
The same is true with us, if we've truly given our lives to Christ. We are now God's children -- and nothing can change that. Sin may destroy our fellowship with God -- but nothing can ever destroy our relationship. The Bible says that nothing "in all creation, will be able to separate us from the love of God that is in Christ Jesus our Lord" (Romans 8:39).
Does this mean it doesn't matter how we live, since God will save us anyway? No, of course not. Jesus Christ gave His life for us; how can we turn our backs on Him? Someone who claims to be a Christian, but denies it by the way they live, may not really be a believer. Make it your goal to live for Christ every day -- and if you do sin, repent and confess it immediately to God, so your fellowship will be restored.