How may we know there is a God?

Answer

The light of nature in man and the works of God plainly declare there is a God (; ; ); but his word and Spirit only do it fully and effectually for the salvation of sinners (; ).

Read the 5 proof texts (KJV)

Romans 1:19-20: ¹⁹Because that which may be known of God is manifest in them; for God hath shewed [it] unto them. ²⁰For 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:
Psalm 19:1-3: ¹The heavens declare the glory of God; and the firmament sheweth his handywork. ²Day unto day uttereth speech, and night unto night sheweth knowledge. ³[There is] no speech nor language, [where] their voice is not heard.
Acts 17:24: God that made the world and all things therein, seeing that he is Lord of heaven and earth, dwelleth not in temples made with hands;
1 Corinthians 2:10: But God hath revealed [them] unto us by his Spirit: for the Spirit searcheth all things, yea, the deep things of God.
2 Timothy 3:15-16: ¹⁵And that from a child thou hast known the holy scriptures, which are able to make thee wise unto salvation through faith which is in Christ Jesus. ¹⁶All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness: