How do you know what the gospel is, what christianity is, what anything is, if you don't know if a religious interpretation is correct. Is the quran harder to read and understand than the gospel that is also translated?. No. It says what it says from Arabic to English maybe a few minor translation errors like the bible has. Everything that has brackets in the quran verses someone has put there.
Jesus said I am the way the truth and the life, is that so hard to understand.
Good questions. You are inquiring into epistemology. As you have pointed out, in the case of Christianity, the term "know" is in the context of knowing a Person, and that Person is the Christ, the True Image of God sent by God. This knowledge comes through the Holy Spirit, and without the Holy Spirit nobody can confess Christ nor comprehend the Gospel. This is why I have said in an earlier post that the Word of God is not a book.
The fact is, that all things are built upon faith concerning what is the Eternal power. And this is because no one can prove that something Eternal is actually eternal, and therefore nor can anyone disprove any accusation that it is not eternal.
What's more, is that that which is Eternal is also therefore Holy and what is Holy is also trustworthy. Therefore faith can be quantified by exploring in what capacity one believes that God/Creator is trustworthy. The kingdom of Light is built upon the belief that He is trustworthy, and the kingdom of darkness is built upon the doubt that He is trustworthy. All semantics in moral/immoral terms including scripture denote one of these two kingdoms.
In moral terminology the highest value is always Love. We know this as an axiom, because Love is the Spirit that is the moral goodness in mankind. We don't need to read about it to know Love, we experience Love so as to know Love. So it is that when we read in scripture where it says
He that loveth not knoweth not God; for God is love, our own spirit testifies to it as fact. Love is therefore the Spirit of Light and where love is absent in various degrees, there is darkness.
I think it's clear that anyone who out of Love would sacrifice their self to save others is trustworthy, and anyone who would sacrifice others to save themselves is untrustworthy. And through the Holy Spirit, this is how I know Jesus is the Christ, and I also know that it's better to believe that Love is Eternal and trustworthy than not to.