How doth it appear that there is a God?

Answer: The very light of nature in man, and the works of God, declare plainly that there is a God;[1] but his word and Spirit only do sufficiently and effectually reveal him unto men for their salvation.[2]

1. Rom. 1:19-20:

   For what can be known about God is plain to them, because God has shown it to them. For his invisible attributes, namely, his eternal power and divine nature, have been clearly perceived, ever since the creation of the world, in the things that have been made. So they are without excuse. (ESV)

   
   Psa. 19:1-3:

   The heavens declare the glory of God,
   and the sky above proclaims his handiwork.
   Day to day pours out speech,
   and night to night reveals knowledge.
   There is no speech, nor are there words,
   whose voice is not heard.

   
   Acts 17:28:

   for “‘In him we live and move and have our being’; as even some of your own poets have said, “‘For we are indeed his offspring.’(ESV)

   
   
   
2. I Cor. 2:9-10

   But, as it is written,
   
   “What no eye has seen, nor ear heard,
   nor the heart of man imagined,
   what God has prepared for those who love him”—
   
   these things God has revealed to us through the Spirit. For the Spirit searches everything, even the depths of God. (ESV)

   
   II Tim. 3:15-17:

   and how from childhood you have been acquainted with the sacred writings, which are able to make you wise for salvation through faith in Christ Jesus. All Scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness, that the man of God may be competent, equipped for every good work.(ESV)

   
   Isa 59:21:

   “And as for me, this is my covenant with them,” says the Lord : “My Spirit that is upon you, and my words that I have put in your mouth, shall not depart out of your mouth, or out of the mouth of your offspring, or out of the mouth of your children's offspring,” says the Lord , “from this time forth and forevermore.”

From the Westminster Larger Catechism.