Even if we didn't have the scriptures we could understand God from nature. This is called "general revelation."

Rom 1:19* Because that which may be known of God is manifest in them; for God hath shewed it unto them.*
Rom 1:20* 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:*

God has given His message through creation.

Psa 19:1* To the chief Musician, A Psalm of David. The heavens declare the glory of God; and the firmament sheweth his handywork.*
Psa 19:2* Day unto day uttereth speech, and night unto night sheweth knowledge.*
Psa 19:3* There is no speech nor language, where their voice is not heard.*
Psa 19:4* Their line is gone out through all the earth, and their words to the end of the world. In them hath he set a tabernacle for the sun,*

God has given His message (Gospel) in scripture. This is called "special revelation." (2 Tim 3:16)

Christ is the Word of God (John 1:14) and the wisdom of God (1 Co 1:24).

If scripture is the word of God, then it shouldn't be different from that given through nature, or else it is another gospel.

Gal 1:8* But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed.*
Gal 1:9* As we said before, so say I now again, If any man preach any other gospel unto you than that ye have received, let him be accursed.*

That's my understanding of it. The proof is philosophical rather than based on history.