The same certainty we have on the Bible and Jesus Christ, other people from other religions have on their gods and "holy books" (between commas because I'm one of those who have the certainty about Bible) and I have no explanation for that and I'd like one so I can answer some friends about it.
Thanks in advance.
This is an interesting question. Many people are firmly convinced that their religious position is, indeed, true. This includes atheistic positions as well. Yet, the question is not with the level of certainty an individual has, but the validity of that certainty. As on writer put it, "faith is only as strong as the object of that faith."
The illustration of ice is mentioned. A traveler from the Arctic regions may visit a warmer climate in winter. While in his home town, they drive cars on the lakes at this point, where he is visiting only seldom as ice thick enough to walk on. Coming upon a lake, he may leap out onto it with all the certainty possible. Yet the object of his faith does not hold up, and he gets very wet and very cold.
Another person from the mild climate may visit an arctic region, and with great timidity, gently step out onto a lake that is covered by two feet of ice. His uncertainty is placed in an object that can well accommodate his act of faith, and he remains dry and on solid "ground".
So, again, faith is only as strong as the object of that faith. As Spurgeon exhorted, "never make a Christ out of your faith." It is not the strength of our faith that saves us, but the truthfulness of salvation that God offers to us through Christ.
So, the real question in this matter is which religious belief is true. All religious beliefs which are false, including atheism, will not support our weight if we put our trust in them. Christianity, however, will indeed support our weight as we put our trust in Jesus, despite the timidity or certainty of our faith.
Christianity alone has overwhelming evidence to support its truthfulness, including hundreds of predictive prophecies fulfilled in Jesus, the inexplicable growth of Christianity in the face of severe and extensive persecution at its very birth, billions of transformed lives, and on and on and on and on... but that's another question.
You are correct, in that all religions all, when measured objectively and independently, essentially have the same level of firm proof, and (give or take) the same level of depth of feeling in the believers (I'm omitting "cultural" associates from this).
This applies both to non-Chistian faiths, and also to the various sects within Christianity - with many fragments entirely convinced of the validity of another competing particular position.
It is interesting. And keep in mind that your belief that another religion is false is exactly what that religion reciprocates - by definition most religions are exclusive.
Put another way: if there was an answer to this, there would be (given our current global communications capability) exactly one religion or zero religions (depending on the result).
As you have observed, a feeling of certainty about religious beliefs is not a reliable indicator to the truth of those beliefs. The big four religions, worldwide, are Christianity, Islam, Hinduism, and Buddhism. (If you wanted to count atheism as a religion, but not merge it with Buddhism, it would probably be between Hinduism and Buddhism.) None have a majority (Christianity broadly construed is accepted by close to 1/3 of the earth's population), and all have sizable numbers of strong believers, advocates, fundamentalists, and so on.
You also can't trust claims of evidence or accuracy or personal benefit. You are already familiar, I take it, with (at least claims of) the extraordinary evidence that supports Christianity. But Islam can claim that many prophecies from the Quran have been fulfilled, and speed of its spread was astounding (far faster than Christianity), among other things. Hinduism is rather amorphous (far more diverse than Christianity), but there are no shortage of miracles (example) attributed to it. Buddhism is an essentially atheistic belief system in that it rejects the existence of God/gods (though it does have some supernatural elements), yet there are many stories of how it has positively changed people's lives.
Therefore, since many of the features of major religions look broadly the same, it must be the details that matter. In particular, if you are talking to friends who (strongly) hold other beliefs, be aware that they are likely to be in a similar situation as you, and if you're going to figure out who is right, you will need to listen respectfully and/or empathize with the feelings their religion may induce in them, while sharing your point of view. If you are talking to friends who hold the same beliefs as you and who are baffled why everyone else doesn't believe the same (maybe it's because they're evil?), you can point out that it's not nearly so obvious as they feel it is, since outwardly all major religions have many similar aspects.
The certainty of religious faith applies to the existence of an ethical standard, which is coming from a moral lawgiver, from God. In logical positivist terms, evidence for the existence of God comes from observing that ethical standards are convergent, so that all people will eventually agree on what constitutes an ethical or unethical act given sufficient time.
This seems like a preposterous notion. But if you ask people of any of the big 5 religious faiths what constitutes ethical or unethical behavior, excluding specific religious doctrines or rituals, they will mostly agree! This convergence is astonishing, especially considering the lack of agreement in earlier times. The religions of South America demanded human sacrifice, for instance.
The functioning of the physical universe is not affected one bit by the existence of an ethical convergence. But in order to get the ethical standards to converge, you sometimes have to tell stories which claim ridiculous things happened, so as to spread the faith. This moral law has led otherwise nice people tell lies. I don't think you should tell lies. God tells me I should not tell lies.
God is not a substance like aluminum. It is a spiritual entity, found by introspection, which informs our ethical judgements. The ethical notion of God is universal to all humanity, and different aspects are revealed in different traditions. All these traditions have elements which are true, otherwise they would have been abandoned long ago. The elements of each faith which are most important slowly become more obvious with time.
No religion can sensibly claim primacy and exclude the others. The ethical principles of other religions are largely compatible with Christianity. They are only incompatible with Christian religious practice. So if you want to practice a religion, you have to choose. But the choice of a valid faith to practice is more like choosing which book you like the best, or which movie you want to see. Its not like choosing the right numbers in a lottery ticket, where all other numbers go to hell.
If you are like me, then you will believe there are no holes in our bible, being the inerrant Word of God, and that makes it easy to defend. But as with other religions and beliefs, you will have to dive in and study them, and find ways to debunk them. As @RexKerr said, you need to do this entirely in love. You aren't going to save souls for Christ without showing them God's love and having empathy for them, understanding, respect, etc., even if they aren't doing the same for you (remember, your love is fed from God, not other people). You will just have a debate that will leave a bitter taste in both your mouths, and will really get you guys nowhere but having doubts in the things that got disputed and will cause both parties to research more to further back up their beliefs, if anything. Also you should have something to say as to where your certainty comes from, and try to be a witness to them (always sprinkling in the message of the Gospel) :)
