Exciting title - huh? Not!
Beginning with Abraham, Creator God took a people who had basically become idol worshipping pagans, and made for Himself a people. Since the time of Abraham and on into today’s Christian / Church age God has been “calling out” from among the nations a people for Himself.
What we read in Scripture is, "Abraham entered into what God was doing for him, and that was the turning point. He trusted God to set him right instead of trying to be right on his own."
If you're a hard worker and do a good job, you deserve your pay; we don't call your wages a gift. But if you see that the job is too big for you, that it's something only God can do, and you trust Him to do it - you could never do it for yourself no matter how hard and long you worked - well, that “trusting-Him-to-do-it” is what gets you set right with God, by God. Sheer gift.
Now think: Was that declaration made before or after Abraham was marked by the covenant rite of circumcision? That's right, before he was marked. That means that he underwent circumcision as evidence and confirmation of what God had done long before to bring him into this acceptable standing with himself, an act of God he had embraced with his whole life.
And it means further that Abraham is father of all people who embrace what God does for them while they are still on the "outs" with God. It is precisely these people in this condition who are called "set right by God and with God"!
That famous promise God gave Abraham - that he and his children would possess the earth - was not given because of something Abraham did or would do. It was based on God's decision to put everything together for him, which Abraham then entered when he believed. If those who get what God gives them only get it by doing everything they are told to do and filling out all the right forms properly signed, that eliminates personal trust completely and turns the promise into an ironclad contract! That's not a holy promise; that's a business deal. But if there is no contract in the first place, simply a promise - and God's promise at that - you can't break it.
This is why the fulfilment of God's promise depends entirely on trusting God and His way, and then simply embracing Him and what He does. God's promise arrives as pure gift. That's the only way everyone can be sure to get in on it, those who keep the religious traditions and those who have never heard of them.
We call Abraham "father" not because he got God's attention by living like a saint, but because God made something out of Abraham when he was a nobody. When everything was hopeless, Abraham believed anyway, deciding to live not on the basis of what he saw he couldn't do but on what God said he would do.
Abraham plunged into the promise and came up strong, ready for God, sure that God would make good on what He had said. That's why it is said, "Abraham was declared fit before God by trusting God to set him right." But it's not just Abraham; it's also us! The same thing gets said about us when we embrace and believe the One who brought Jesus to life when the conditions were equally hopeless. The sacrificed Jesus made us fit for God, set us right with God.