Grace and peace from God and from the Holy Christ!
Because it is my work of the faith of God to do:
John 6:28 Then said they unto him, What shall we do, that we might do the works of God?
John 6:29 Jesus answered and said unto them, This is the work of God, that ye believe on him who he hath sent.
Does it mean that Satan who believes and trembles does the work of God? No! Any man can believe anything he wants... but a man who believes because he has obtained the faith of God
(which Satan cannot obtain) does the work of God that comes with faith.
When we believe God because we have obtained the faith of God, as Abraham did, we please God. When we believe God or anything else having not received the gift of faith, we do not please God and God does not respect anything we do from such wayward believe.
Cain is one scriptural example of a man who believed God without faith. Abel, his brother, believed and obeyed... and it is recorded in Heb. 11 that by faith he
(Abel) pleased God.
In addition to believing, the faith we have obtained also imputes the Spirit
(with which we are sealed), the power
(from the word of truth that promises), grace
(the pool of riches from which God promises all good things), righteousness
(the work that Jesus Christ did as the propitiation for our sins), and the oath (
all promises of God to heirs of promises - faithfuls - are backed by an oath).
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Believing and obeying constitute what is called the
"preparation" of faith. Those are works of faith in the absence of which faith is dead
(James 2:17) and by which the measure of faith increases or decreases.
In summary, I believe because it is my obligation unto God and I must do and finish all works of God
(which includes believing God):
Ephesians 2:10 For we are his workmanship, created in Christ Jesus unto good works, which God hath before ordained that we should walk in them.