"It is written" is one of the most important messages from God. The reason is, men and women have the ability to make up their own truths, create their own religious philosophy, make up their own belief system, even create a completely false set of values and morays. But God, in His infinite wisdom, along with creating all the universe and you and me, decided to make sure that there could be no question about all that is necessary for salvation and a godly experience in this life. God did that by creating His spoken word into the written word, using willing, inspired vessels, men and women just like you and me. God still uses men and women to write down inspired words of wisdom and truth that will assist others in finding their true self and experiencing a better life with God.
If you are skeptical about the written word of God, the Bible, then read it. Pray about what you read and let faith have its place in your life. Whatever faith you give to the scriptures will be equally rewarded with belief in those scriptures. Faith turns words on the page into belief that brings things to pass. Can any other book do that? No. The Bible is the only codified, inspired and validated book from God ever written. It is validated through exposure to history without fail and by personal experience and confirmation by the Holy Spirit within. Any attempt to discredit the scripture is futile, because God cannot help but be God and when God had the words of scripture written down, He verified every word so that it could be trusted from age to age without fail.
Don't be sucked in by all the new-agers that believe anything or by other religious philosophies that purport that they are on equal ground with the Bible. If it were so, the Bible would make it clear that there are other sources to qualify your eternal destiny. But there isn't. That's why Jesus can say without any doubt, "it is written," and know that if it is written by God it is as good as if God were standing in the room.
I wish we had believers like that today. When they read the word of God they believe it and move on it and respond to it and follow it, that same as if God had physically given them a command, standing in the same room. Just for the record, that's really the only kind of faith that gets results; the I believe what God says in spite of what I see with my eyes or hear with my ears, despite the outcome, despite anything."