I remember reading in one book about faith: "How do we know that God is for us? What do we do or how to response to that word "if" in Romans 8: 31? We believe that God is for us because He chooses to."
There is no assurance in this arbitary "He chooses to."
Why does He choose to? And what is our proof of this love?
John records Jesus' own answer to both of these questions:
"For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life." (John 3: 16)
God so loved the world -- that includes you and me -- that He gave His only Son, the Son whom He loved.
We know that God is for us because He gave the best to redeem us, even when no one was paying attention, and we were lost in our sins:
"But God commendeth his love toward us, in that, while we were yet sinners, Christ died for us." (Romans 5: 8)
God does not arbitrarilly forgive us of our sins and grant us new life in His Son. It is based on a righteous, judicial foundation:
"For Christ also hath once suffered for sins, the just for the unjust, that he might bring us to God, being put to death in the flesh, but quickened by the Spirit:" (1 Peter 3: 18)
"For he hath made him to be sin for us, who knew no sin; that we might be made the righteousness of God in him." (2 Corinthians 5: 21)
Today, we can come boldly to the throne of grace because of Jesus, our high priest forever. We do not have to wonder if God the Father is for us, because He sent His Son to die for us. the proof positive which testifies that He is good and loves us:
"If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness." (1 John 1: 9)
He is faithful and just to forgive us our sins.