A rational person could reach the conclusion that Christianity is "probably" true...However, is there any way for a rational person to reach the conclusion that Christianity is "definitely" true?
I submit that such a transition is, logically, entirely dependent upon the existence and reception of supernatural revelation.
Supernatural: of or relating to an order of existence beyond the visible observable universe
The answer to “Does God exist?” has only two possibilities: yes or no. And the answer to that question is what it is no matter what any of us believe because that's how existence works: It's binary. Any particular thing either does or does not exist.
Starting with the definition that "God" truly transcends the material universe (that God is supernatural) and with the assumption that the material is ignorant of the supernatural except by some revelation, the honest atheist has no logical basis for claiming certainty of God's non-existence. Prominent atheist Richard Dawkins has said as much in his book The God Delusion:
“I cannot know for certain but I think God is very improbable, and I live my life on the assumption that he is not there.”
The uncertainty comes because a non-existent thing cannot reveal it's non-existence absolutely. Now if we limit the parameters, for instance, “There is an Indian Elephant existing in my back left pants pocket.”, then the assertion breaks down and certain disproof is possible. However, since even the now famous Flying Spaghetti Monster is essentially non-disprovable, certainly something as transcendent as God (capital G) would be non-disprovable and would have to self-reveal for us to perceive.
In fact, we know virtually everything by revelation. Our environment reveals it's existence to us, is received through our senses, and patterns are processed in our minds (here is logic and science and all rational thought). If a blind man is severely hampered in his physical perception of physical revelation, how much more are physical beings hampered in their perception of the supernatural (if it exists).
If God does not exist then all theists are wrong no matter how certain they appear (and indeed some of them might be nuts because this is not a logical certainty) and all atheists (again, the honest ones) are correct but unsure. A God that does not exist cannot reveal His non-existence and so the honest atheist and the honest theist must both remain uncertain. Incidentally, this is why there are probably only agnostics and theists.
If God does exist then there are two possibilities: He has either revealed Himself or He hasn't.
If He hasn't revealed Himself then the atheist is wrong but must remain unsure for all the same reasons listed above, while theists make a correct claim all the while any certainty is illogical because no actual revelation has been given. Everything here is the same as above except for which party is unknowingly right and wrong. Keep in mind we are talking about the yes or no of God's existence and not the abounding variety of theologies about God.
If God has revealed Himself, there are two more possibilities: We either receive the revelation and think accordingly or we do not.
a) The atheist in this scenario operates apart from the revelation (if that is even possible) and he must, again, remain unsure. The person who has but rejects revelation would fall into a different category. This person does actually have ground for certainty but dislikes the implications and any claim at all has no logical ground.
b) The theist, however, now has logical ground for a claim of certainty. He might be making a false claim while actually in non-reception or rejection of revelation but...the theist might actually know for certain: It's logically possible.
And this is the foundational Christian claim:
He that hath received his testimony hath set to his seal that God is true. - John 3:33
There are those within "Christianity" who espouse uncertainty regarding the most basic tenets of theism. These will say that belief in God cannot ever attain certainty "this side of heaven" and they will say that if such certainty were possible then "faith" would be needless.
However, no one who has actually been born from above, who has actually been translated from the kingdom of darkness to the kingdom of light and from death to life, and who has actually been given the holy spirit of promise all based upon the reception of the testimony of Jesus can be other than certain of at least the most basic things.
A Christian can logically be certain that God exists, loves them, and has forgiven them their sin in Christ Jesus for the simple reason that, when that revelation is received for what it is, the Godhead actually takes up residence within them:
At that day ye shall know that I am in my Father, and ye in me, and I in you. He that hath my commandments, and keepeth them, he it is that loveth me: and he that loveth me shall be loved of my Father, and I will love him, and will manifest myself to him. Judas saith unto him, not Iscariot, Lord, how is it that thou wilt manifest thyself unto us, and not unto the world? Jesus answered and said unto him, If a man love me, he will keep my words: and my Father will love him, and we will come unto him, and make our abode with him. - John 14:20-23
According to strict logic such a Christian with such a claim of certainty is either likely insane or correct. According to Biblical revelation such a claim of certainty is to be expected:
If we receive the witness of men, the witness of God is greater: for this is the witness of God which he hath testified of his Son. He that believeth on the Son of God hath the witness in himself: - 1 John 5:9-10a
The impartation of such certainty to another one is entirely based upon that one's reception of the same testimony. For the Christian this constitutes the entire necessity of preaching the gospel in word and in deed.
