I'm certain, but in interfaith it's just more tactful to call it belief. This planet is a big place and there should be room for everyone. People who state their beliefs as facts don't understand what a fact is.
Ok, you're certain about your belief in reincarnation but other Hindus have differing beliefs. And likely they are certain, too. It's possible that the atheistic Hindu is certain about his belief, too.
Like everything else, restrictions in Hinduism too are limited to what a person believes. That is why I inserted this condition. There are people like the famous 'Ramakrishna Paramhans' who became a Christian and a Muslim for a fortnight each, who will bow to Jesus and Allah. However, my views are more restrictive. I do not like any mix-up between Hinduism and Abrahamic religions; though other Indic religions, Jainism, Buddhism and Sikhism are OK, even Dao is.
Absolute certainty about one particular God, as if they had any proof of it, and to the exclusion of all other deities worshiped by other people (Only my books is correct, all others are false; only what I say is correct and others are fools; even when it appears in Hinduism), to me, is abhorrent.
So then it could be, or maybe it isn't, is that right? There is an old story that some would tell here (U.S.) and it goes something like this:
There was a man who really wanted to know the meaning of life. And he was very rich, so at a cocktail party he was having, he asked all his guests if they knew what is the meaning of life. Only one guest was able to help him. He told him about a man he heard about who lived at the top of a mountain, he was a hermit, but came out of his cave from time to time. If anyone knows the meaning of life, he said, that man must. But you will have to give up everything to get there, it is a long and arduous trip and you may not return. The man said, "Yes! I must go!" And so he gave up everything to get there. He hired guides, and after much time, made his way across the world and climbed the mountain to meet the ascetic. He saw him at last outside his cave, sitting and meditating. And he went over to him and said, "Oh, great sage, I have given up everything to ask you, tell me, great sage. You surely must know. WHAT is the meaning of life"? And the hermit said, holding up one finger, "Life is like a mountain." "Life is like a mountain?" the traveller said? "Life is like a mountain"? "Well, the hermit replied, "Maybe it's not like a mountain."
