I believe in Once Saved Always Saved, but I hear from some people that when you are a mature believer, you have the choice to “leave the house” and forfeit your salvation and be damned. God is not going to let an immature believer leave the house, since he does not know any better, but when he or she knows about departing from God, he can make the choice and go to hell. Does the Bible truly say when you are a mature believer, you can walk away and be damned?
The Bible makes no such statement. Think about it. A mature believer is someone who knows what they believe and why they believe it. They’ve acquired Bible knowledge and understand the concepts of both eternal life and eternal punishment. Do you really think that such a person would consciously reject eternal life in favor of eternal punishment?
But more importantly, the Bible clearly argues against this preposterous notion. For instance in Ephesians 1:13-14 Paul said our salvation was guaranteed from the time we first believed and the Holy Spirit was sent to us as a seal marking us as God’s possession. 2 Cor. 1:21-22 confirms this saying in effect that God has taken ownership of us and it is He who makes us stand firm in Christ. In 1 Cor. 6:19-20 Paul said we have become a Temple of the Holy Spirit and are no longer our own, but have been bought at a price. In John 10:27-30 Jesus said no one can take us out of His or His father’s hands. And in Romans 8:38-39 Paul said no power in Heaven or on Earth can separate us from the Love of God. These passages make it clear that once we become believers we belong to God and can no longer choose to depart from Him.
Therefore, a person who walks away, or “leaves the house” as you put it, never belonged to the house in the first place. John said if they had belonged to us they would have remained with us, but their going showed that none of them belonged to us (1 John 2:19).