What are the major problems with the Free Grace teaching that you stay saved even if you stop believing?

This is one of those debates where I wanted to reach across and say, brother, you are in heresy and you are sending people to hell, but it was a professional debate, so I did not. He was building a case, using John 6:47, that you cannot lose your salvation even if you stop believing, because he pivots everything on the first act of belief: you first believe, that is as good as your name written in stone, and even if you go on to sin like crazy and do not believe in Jesus anymore, you are still saved. To me that is as heretical as you can get, and the devil loves that idea. When we got to 2 Timothy 2, where it says if we disown him he will disown us, because he must be true to himself, he read that as meaning Jesus has to honor your one act of belief forever, which is assuming his premise, not proving it. So I went to 1 Corinthians 3, where it talks about building with wood, hay, and stubble or gold, silver, and precious stones, and God's judgment: verse 15 says a man can be saved yet so as by fire, which is where the Catholic Church gets purgatory, but verse 17 says if you destroy the temple of God, God will destroy you, and that destruction has to mean something worse, namely hell. At least those who believe in faith alone believe they must have faith until they die; these Free Grace people say all you need is to believe once, when you were seven years old, and then live any way you want until you die. What a heresy that is.