John 6: 63
The spirit is what gives life; the flesh profits nothing; the words that I have spoken to you are spirit and they are life.
Source
The word of God is a spiritual food for our lives and is the defense of our actions along our path. There is power and promise in the word of God.
Jesus was very clear in expressing his defenses to the enemy when he said "because it is written", there the Lord was categorically sustaining the power of the written word. For example, we can read in the book of Matthew chapter 4 when satan tempts Jesus and his response at all times was "because it is written".
It is important to point out that from the divine point of view temptation constitutes positive proof, from the point of view of the devil it implies an incitement to sin; for Jesus it was a challenge of Satan to prove the sovereignty and plan of God.
So this expression of Jesus was taking for granted that in the word of God there is no doubt but that it takes for granted a fact and must be interpreted as truthful and safe because "it is written" and this must be a conviction for the Christian.
In that sense God gave us his word to be our spiritual food and nothing could be more important to us than to nourish ourselves on the word of God to have the right word when applying it.
We must remind the enemy each time he uses someone who is wrong because you have promises "because it is written".
God said it and against such things there is no law or buts that is worth.