People often criticise Christianity as irrational since they allege that you can't actually experience Jesus as an historical person or even prove the existence of Jesus's miracles, resurrection etc.. (Jesus as an historical person, though, is normally accepted and can be proved in the same way as the existence of Caesar or other characters from about that period are proved.)
So, is it irrational to believe in something that you can't directly observe?
Certainly, most things we believe in, we believe in from experience. However, there are things that we understand/believe in from logic etc. - eg. people normally accept evolution as rational even though it can't be directly observed (discounting the use of fossil evidence (for what it's worth) since it suffers the same problems as the historical evidence of Jesus and his activities).
However, most would agree that even though processes such as evolution occur too slowly to be observed, the results of the processes may be observed.
On this basis, are there any "results" which are able to act as evidence for the alleged activities of Jesus and that He is the Son of God?
The behaviour of the first disciples and the subsequent formation of the Christian Church is a good effect to consider. Many of the early Christians, who knew Jesus (such as the apostles), suffered greatly for what they believed. Many even died due to their faith. If they felt that Jesus was fraudulent or misguided, it is highly unlikely that they would have done so.
There was no shortage of people claiming to be the Messiah around the time of Jesus, but their followers dispersed and soon disappeared, particularly following the death of "believers". That this did not occur in the case of Christianity itself gives pause for reflection.
(By way of aside, pointing out that the followers of, say, David Koresh also suffered greatly and died too isn't really an answer. There is, of course, a significant difference between the followers of Mr Koresh and the early Christians. The former died with their leader. The latter suffered and died after the death of their leader who Himself had claimed that He would rise from the dead, and many of them died having claimed to have seen Jesus alive after He had been buried. I don't think Mr Koresh is recruiting many more followers, and the survivers don't seem that keen to proselytise.)
Some people take the view that "All roads lead to Rome" or that "How do you choose a religion when there are so many claiming to be right?"
Indeed, there are numerous religions in existence and most of them have mutually exclusive claims. Also, most of these religions are more than transient fads.
But the claims for Christianity are strong.
I could start a religion which may persevere from now until the end of the world, so long as I can substantiate the claims upon which I base that religion. On the other hand, if I make claims which are readily refuted, it is unlikely that the religion will last very long.
What this means is that I could start a religion which believed in re-incarnation and that the soul was on a journey through many lives to a higher plane and many people may well believe it and it may well prosper.
On the other hand, were I to claim that I was God's son and would rise from the dead and I didn't actually rise from the dead, the whole thing would be a complete joke - the religion wouldn't take off.
Furthermore, the fact that there are several "competing" religions does not prove that they are all false (or that any one of them is false).
Other people argue that, if there is a god and he (she/it) condemned them to hell for not believing, then that god is "nasty" for not personally proving himself to the people and is not worthy of worship.
In Christian theology the reason people wind up in hell is because they refuse to acknowledge God as their creator and consequently His right to be the one who decides how they should live their lives (ie. it is more than simply because someone didn't believe).
Ultimately the fact that someone believes that what God has done isn't good enough for them is itself (in Christian theology) a further indication of their rebellion against God and their refusal to acknowledge him.
Some people go further and argue that, if God does exist and is "omnipotent", then He could surely come up with something to convince them of His existence.
But, why should God bother providing people with enough evidence to believe in him? Is that His job?
This response also neglects the fact that we do have good evidence (largely in the form of Jesus).
Often scientists (and people of that bent) like to test theories by asking whether they can be used to predict outcomes. Christianity can also be tested in that way through looking to whether any of its prophecies have been fulfilled. There are lots of examples of fulfilled prohecies (a good one is Isaiah 52-53 which was written many years before Jesus was even born).
As an aside, I believe that we only have the information about God that we do by the grace of God.
The corollary of this is that it is because we live in a world alienated from God that we don't have more abundant evidence. Part of God's judgment on this world is that He leaves us to our own devices.
This shouldn't be considered a rationalization of the lack of evidence. This can best be explained by example.
Prior to the current proof for the existence of atoms, scientists could explain that there is not a lot of proof because the particles are very small etc. and it is hard to test their existence. If someone then responded with:
"By explaining why there is inadequate evidence you're just rationalizing the problem away - if atoms exist they ought to provide more evidence for their existence!"
At worst, the appropriate response should be:
"I don't know if atoms exist."
What I'm saying here is that if God appears to be inaccessible then there ought to be some explanation for that. If there is a valid reason why we might not expect evidence, then it ought to be accepted.
It would be a whole other matter if there were no such reason, or indeed if I expected there to be more evidence for God in existence than there actually is.
But, I do still stand by the proposition that there is adequate evidence for belief in any case!