God has chosen to reveal Himself to mankind in more than one way. God has revealed Himself through His work of creation known as general revelation. The Lord has also revealed Himself through special revelation, which includes His appearing to certain individuals as found in scripture. Special revelation also means that He made Himself known through His Son Jesus Christ and through Holy Scripture.
"The heavens declare the glory of God; and the firmament sheweth His handiwork." -Psalm 19:1
"For the invisible things of Him from the creation of the world are clearly seen, being understood by the things that are made, even His eternal power and Godhead; so that they are without excuse." - Romans 1:20
"In the beginning was the Word, and the Word was with God, and the Word was God...And the Word was made flesh, and dwelt among us, (and we held His glory, the glory as of the only begotten of the Father,) full of grace and truth." - John 1:1, 14
The Bible is God's unchanging word to an ever changing world. It was written by human authors who were guided and guarded by the Holy Spirit. The Bible is the absolute standard that gives direction and instruction for the Christian's life and practice. Since it is given by inspiration of God, it is without any mixture of error.
"All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness: that the man of God may be perfect, thoroughly furnished unto all good works." - 2nd Timothy 3:16-17
"For the word of God is quick, and powerful, and sharper than any two-edged sword, piercing even to the dividing asunder of soul and spirit, and of the joints and marrow, and is a discerner of the thoughts and intents of the heart." - Hebrews 4:12