But how do you know that it's true?
It testifies of itself.
Truth is by it's own nature true. In other words, true=true and false=false.
As far as an epistemology, quite simply it is revealed.
For example:
John 16:13
13 But when he, the Spirit of truth, comes,
he will guide you into all the truth. He will not speak on his own; he will speak only what he hears, and he will tell you what is yet to come.
The means through which that revelation comes is scripture.
2 Timothy 3:16
All Scripture is God-breathed and is useful for teaching, rebuking, correcting and training in righteousness,
The unregenerate, those without the spirit of God such as atheists, Muslims, and many who claim to be Christian do not discern what is spiritual, nor can they.
1 Cor. 2:14
The man without the Spirit does not accept the things that come from the Spirit of God, for they are foolishness to him,
and he cannot understand them, because they are spiritually discerned.
How do we know for absolute fact that our way of thinking it the only truth?
Two things here:
In one sense, you can't know absolutely, nor can you know anything absolutely (including the statement). Which is not a problem regarding scripture, since scripture says we live by faith.
Second though, we can be assured that what scripture speaks of is truth revealed by God due to the fact that it
isn't our way of thinking.
Isaiah 55:8
"For my thoughts are not your thoughts, neither are your ways my ways," declares the LORD.
You also can assess other truth-claims by their own standards, and they fail due to internal incoherence. Christianity is the only truth claim that stands on it's own.
Yes, the Bible says so, but man wrote the Bible.
Irrelevant.
Proverbs 30:5
"Every word of God is flawless; he is a shield to those who take refuge in him.
Yes, God inspired it, but it's not the direct handwriting of God Himself, so how can it be absolute truth?
See Proverbs 30:5.
It is God's word, it is flawless.
Now, specific translations have issues. The autographs do not.