Joh 3:12 If I have told you earthly things and you do not believe, how will you believe if I tell you heavenly things?
The are two types of revelation in this vs. Natural/outer (Earthly things) that helps men see that God has created everything and Supernatural/inner (Heavenly things) the mystery of the Gospel. Psa 19:1 “The heavens declare the glory of God; And the firmament shows His handiwork“, Rom 1:20 “For since the creation of the world His invisible attributes are clearly seen, being understood by the things that are made, even His eternal power and Godhead, so that they have no excuse” (both are natural revelation). People are born with natural revelation manifested at the age of awareness, however God never chose the natural revelation to be the means to save men but to prepare men for the supernatural or special revelation. The natural revelation may create a desire for special revelation.
The voice of God is His word, this is the special revelation. “Faith comes by hearing the word and not by studying the nature. Nature drives us to scripture for true understanding of God and His ways, it’s the word of God that gives birth to faith” (Phil Johnson).