"And when the tempter came to him, he said, If thou be the Son of God, command that these stones be made bread.
But he answered and said, It is written, Man shall not live by bread alone, but by every word that proceedeth out of the mouth of God. " - Matthew 4:3-4
It is written. Implies something that is written down. Let's look at the verse in question as referencd in the OT:
"All the commandments which I command thee this day shall ye observe to do, that ye may live, and multiply, and go in and possess the land which the LORD sware unto your fathers.
And thou shalt remember all the way which the LORD thy God led thee these forty years in the wilderness, to humble thee, and to prove thee, to know what was in thine heart, whether thou wouldest keep his commandments, or no.
And he humbled thee, and suffered thee to hunger, and fed thee with manna, which thou knewest not, neither did thy fathers know; that he might make thee know that man doth not live by bread only, but by every word that proceedeth out of the mouth of the LORD doth man live." - Deut 8:1-3
So it is referencing the commandments as laid out in the Torah, the Law of Moses.
Jesus summed up the Law in two commandments in Matthew 22:37-40:
"Jesus said unto him, Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind.
This is the first and great commandment.
And the second is like unto it, Thou shalt love thy neighbour as thyself. On these two commandments hang all the law and the prophets."
I would not have you overlook the importance of this last statement, for it is vital for us to realize how the Word of God is really enacted in our lives.
Why was Jesus referred as the Word of God? Because He live perfectly according to the commandments of God. He embodied the Law. And He was able to keep the commandments not by following the letter of the Law, but by being led by the Spirit of God. The very essence of God's Spirit is the key to fulfilling the Law.
"But if ye be led of the Spirit, ye are not under the law....
But the fruit of the Spirit is love, joy, peace, longsuffering, gentleness, goodness, faith,
Meekness, temperance: against such there is no law." - Galatians 5:18, 22-23
I would submit to you that the Word of God is spiritual. It is commandments written in the human heart through the Spirit of God.
"A new heart also will I give you, and a new spirit will I put within you: and I will take away the stony heart out of your flesh, and I will give you an heart of flesh.
And I will put my spirit within you, and cause you to walk in my statutes, and ye shall keep my judgments, and do them." - Ezekiel 36:26-27
What we have here in the document we call the Bible is an imperfect representation of the spiritual Word of God that is Perfect. Much like the Tabernacle is representative of the Tabernacle not made with hands, the Bible is pointer to that which is spiritual.
That is why the Bible can be imperfect, but useful to pointing us to the Perfect. The truth is there, but it must be experienced.