Dear Mr. Ahsan,
We can not change established definitions. Buddha never rejected
God. He also did not say that he was a God appointed prophet. To me,
he was a social and spiritual reformer. He was not an atheist. His
followers never reject the existence of God.
Besides, you have written that how the Marxists and atheists are
non-believers? According to established norms and definitions who so
ever does not believe in God is a non-believer, hence they are
non-believers. If you insist on giving them the epithet of
believers, then you can do so by saying that they believe in no
belief. It will be an interesting addition to religious terminology.