The faith that saves as stated in Eph 2:6-10 is better understood if we get to the very root of what the Bible is referring to when it so frequently mentions faith or its absence, even in the OT. It starts in the Garden of Eden and Eve's temptation, Adams sin. That is the very place faith in God was broken, and was passed on to us all. It is more why they disobeyed God's command, than the simple fact that they did. In Gen 1 we see God creating the earth and all that is in it and pronouncing it good meaning perfect, self sustaining through the wisdom and power of God, and supplying every need for both vegetation, animal, and man. In verses 26-31 we see God creating Adam and giving him dominion over the earth and all that was on it. (We see too the complete absence of death or violence, even in the animal kingdom against one another and us against them, them against us.) In Gen 2:16 God gave a command to Adam. And the Lord God commanded the man, saying, "You may surely eat of every tree of the garden, 17 but of the tree of the knowledge of good and evil you shall not eat, for in the day that you eat of it you shall surely die."
In Gen 3 we read of Eve's being deceived and we see why she ate. 6 So when the woman saw that the tree was good for food, and that it was a delight to the eyes, and that the tree was to be desired to make one wise, she took of its fruit and ate, and she also gave some to her husband who was with her, and he ate. Whereupon they realized they were naked and immediately tried to hide from God, and cover the evidence their own disobedience. God in His mercy, supplied them with a better covering, one made from the first death to enter creation. The beginning of a life for a life that ends with the sacrifice of Jesus.
So what happened here? Even though everything was perfectly provided for them by God, and He walked with them, His presence with them, they failed to trust Him when something they didn't have was forbidden. They wanted that thing.
Everytime the Bible speaks of persons in the OT as having faith, or having a heart after God, it is speaking of those who put full trust in God to be who He says He is and to do what He says He will do. That was the difference between David, a man after God's own heart, and Solomon, who God said was not as his father David. The difference? David never failed to trust in God and their covenant relationship with its promises. Solomon, even though his reign as king began right and he was blessed with wisdom and prosperity and peace, eventually wanted other things too. Other gods, forbidden things, and went after them. He was trusting in other than God. No faith that God was all He needed or wanted.
The faith that saves is the same type of faith. Jesus had it, as Son of man, trusting in the Father. Other faithful men, sinned, stumbled, repented, but did not lose faith. Jesus never stumbled in His faith in God to do what He said He was going to do, nor did He ever sin against Him. It was this trust in God, that even though Jesus was going to have to suffer unimaginably, even to the point of God forsaking Him, that God would raise Him to life again; and that He was doing it for a people that God was giving to Him to redeem from the power of sin and death that came in Eden. He did it for the Father, and He did it for those people. He trusted God to do what He said He would do, and so obeyed Him at every point. It is this faith of Christ, through absolute trust (faith) in Jesus to be who He says He is, and to have done what He said He would do, and to do what He says He will do in the future, that is the faith that gives eternal life through forgiveness of sins. We will never find this faith within ourselves, we are too sinful and fallen to do so. It is given by God and by His grace. He gives us the momentous faith of the Son and imputes to us, His perfect righteousness, and we believe.
And though we will not find this faith within ourselves, we can and should ask for it.
