When we come to Christ, we have a new relationship with Him: Instead of being His enemies, we're now His sons and daughters! The Bible says, "The Spirit himself testifies with our spirit that we are God's children" (Romans 8:16).
This is a very precious truth - but it also helps us understand what happens when we sin. Think of your own family for a moment - perhaps the family you grew up in, or your family right now. What happens when someone in your family does something wrong? You know what happens: The closeness of your family is strained, and the fellowship between the wrongdoer and the rest of the family is broken. Only when they ask for forgiveness can the relationship be healed.
But when they do wrong, do they cease to be a member of that family? No, of course not. The fellowship may be broken - but not the relationship. No matter what they've done, they will always be a member of that family.
The same is true for us. If we have truly given our lives to Christ, we are now God's children - and nothing will ever change that fact. But when we sin, our fellowship with God is broken - and that's why we need to confess our sins to Him and seek His forgiveness without delay. The Bible says, "If we confess our sins, he is faithful and just and will forgive our sins and purify us from all unrighteousness" (1 John 1:9). Make this your practice every day.