1 Peter 1:21 "Who by Him (that is, Christ) do believe in God, that raised Him up from the dead, and gave Him glory; that your faith and hope might be in God."
Why is belief in God by Him?
First, because the chasm between the greatness of divinity and the smallness of human capacity makes the knowledge of God impenetrable. Belief in God apart from His self revelation end in imaginary thoughts about what God should or shouldn't be. The Old Testament is a written, historic record of God self-revealing. The New Testament is a written, historic record of God in Jesus Christ (Hebrews 1) as "flesh and blood" to mediate and "break the power of death" (Hebrews 2:14-15). Jesus Christ is the "image of the invisible God" (Colossians 1:15). In essence, God made Himself very small (Philippians 2:5-7) to accommodate Himself to human capacity.
Second, just as the fall of humankind separated them from God eternally, belief in God by Him unites people to God in love eternally. By nature everyone hates the God that has self-revealed as a Holy consuming fire of justice because they love sin. Hatred of God and love of sin necessitate the invention of gods with characters more 'acceptable' to the sinner so that some level of spiritual 'goodness' is attributed to human character without having received the power and intervention of the Divine. These inventions, in part or whole, are designed to avoid the pain of self-realization as "dead in sins" (Ephesians 2:1) and the necessity for total submission to a Savior (John 14:6).
That's why belief in God is by Him. The human race makes much ado about God and gods, but often not by Him - the Savior. Everyone has religion, whether true or invented, but no one can have salvation by an invention of self or some other human. Belief in God or gods apart from Jesus Christ sometimes has "a form of godliness", but always "lacks the power thereof" (2 Timothy 3:5). Belief in God by Him is the only belief with the power to save.
