Christians may offer many different sorts of arguments for why they believe in Christianity but usually they are often just fart-logic (see definition here). You see, the actual reason they know Christianity is true is because that of what that quite little voice in their heart tells them.
A leading Evangelical theologian, William Lane Craig, confirms exactly that:
“The fundamental way in which we know that our faith is true is through the self-authenticating witness of God’s Holy Spirit. That is to say, God himself, in personal relationship with us, bears witness to us that we are Children of God and we are rightly related to him and that Christianity is therefore true.”
— William Lane Craig
See the video of Craig at “Common Sense Atheism” where Luke explains this better than I do (as always). Remember, all the reasons Christians give for WHY they believe are usually just fart-logic in that as soon as you shoot down whatever reason they give, they desert that explanation and run to yet another. And even if you shoot down all these fart-logic arguments, they still believe. That is because the real reason they believe has nothing to do with the reasons they state. Their stated beliefs are mere post hoc justifications for usually very simple reasons.
All religions base their faith on the feeling they get inside, so Christianity is no different that all the rest. Further, Craig used the phrase “Personal Relationship” which is used extensively by evangelicals. I have written already on how this phrase is totally manipulated and misused by Christians.