Is it true? No.

Yes, we do know. Jesus told us.

Jesus said it plainly that the only sin that can’t be forgiven is blasphemy against the Holy Spirit. But what is that? Since the Bible interprets itself, we have to look at the context. The Pharisees were saying that Jesus did works of the Holy Spirit by the power of the devil. So, blasphemy of the Holy Spirit would be to attribute the works of the Holy Spirit to the devil. Very dangerous!

Truly I tell you, people can be forgiven all their sins and every slander they utter, but whoever blasphemes against the Holy Spirit will never be forgiven; they are guilty of an eternal sin.” He said this because they were saying, “He has an impure spirit.” Mark 3:28-30