Actually, they don't KNOW this for fact. They rely on faith. Christians rely on the assumption that the Bible was faithfully handed down through the ages, was not made up by man, and is interpreted for their sake by honest people. My experience with the clergy has greatly shaken my trust in them, and conseqently the entire shebang.
It is fortunate that this is really in God's hands rather than our own.
Actually, they don't KNOW this for fact.
Sure they do. They KNOW IT -- as much as I "KNOW" that Obama exists. I get that from the testimony of others.
I'm told that this funny little box in my house actually conveys "pictures" of a real person to me and that I can "see" this person "Obama" -- but I've never talked to him, I've never seen him in person -- but I've only read about him, and heard what other people have said about him. And I've read that some other people "have actually seen him".
That's another thing that I KNOW works -- is that funny little box that takes pictures from one location and carries it over to me and shows me something that I couldn't see "in person". I have only taken "other people's word" on the matter, as I've never tested out that theory to see if it works. Other people have apparently tested it out, but I haven't. BUT, I KNOW that it's true, even though I haven't tried it out.
SO ... I KNOW that Obama exists, even though I've never "tested that theory" in practice at any time in my life.
BUT, I also KNOW that George Washington existed, and I don't even have that funny little box with pictures on it to "show me" about him. In that case, I only have references from other people who assure me that those "documents" that say George Washington existed, really are true. I don't know that from having somehow tested out that idea for myself -- and I'm not quite sure how I could "test it out" by mathematics and the laws of physics right now to prove to me that George Washington existed, anyway.
However, I do KNOW that George Washington existed, because I do think the "testimony" of many others (some who saw him in person at the time and who have written about it) are true. So, I do KNOW that George Washington existed.
BUT, come to think about it -- all those things that I KNOW TO BE TRUE, in my ordinary every day life, most of it I've never tested out, I've never seen for myself and I only accept "testimony" from others that it's true.
The only thing about that, is that I've determined that most of it is "reliable testimony" -- so I KNOW that these various things are true from that reliable testimony about many things in life that I cannot directly verify.
SO..., YES ... Christians do KNOW these things to be true ... from the reliable "testimony" from God and His written word. :-)
This post brings to my mind what Paul said....(not quoted)....that he only wanted to know Christ and Christ crucfied. Simple gospel and that’s it. God sent His son for us, To save us from ourselves and what is to come..
And if you boil down Christ’s teachings to love God with all your heart and soul....well that will then lead to most doing likewise with their fellow man.