And he believed in the Lord; and he counted it to him for righteousness. (Genesis 15:6)
“Therefore, we do not say that Abram was justified because he laid hold on a single word, respecting the offspring to be brought forth, but because he embraced God as his Father. And truly faith does not justify us for any other reason, than that it reconciles us unto God; and that it does so, not by its own merit; but because we receive the grace offered to us in the promises, and have no doubt of eternal life, being fully persuaded that we are loved by God as sons.” – Calvin, Commentary on Genesis 15:6
…[Abraham] was fully persuaded that God was able to fulfill, and would certainly fulfill, the promise made to him concerning a child, and especially concerning the Messias, who should come out of his loins by that child, and that both himself and all people should be justified and blessed in and through him. – Matthew Poole, Commentary on Genesis 15:6
Some think that his believing in the Lord respected, not only the Lord promising, but the Lord promised, the Lord Jesus, the Mediator of the new covenant. He believed in him, that is, received and embraced the divine revelation concerning him, and rejoiced to see his day, though at so great a distance, John 8:56. – Matthew Henry, Commentary on Genesis 15:6
The belief of Abraham takes a prominent place in the theology of the New Testament. In Romans 4, Paul explains that Abraham’s circumcision meant nothing for his salvation. Moreover, the faith imputed to him as righteousness came to him when he was outside the covenant of God, at least as the Jews would have identified him since he was uncircumcised.
What was the content of his faith? Calvin suggests it was that he “embraced God as his Father.” Certainly, as believers united to Christ, we receive the Spirit of adoption as sons (Romans 8:14-15) which leads us to embrace the Father. Yet, Poole and Henry see here that Abraham believed in the promise of the Messiah, of the Christ. The content of his faith is no different than ours, we only have revealed in redemptive history what was promised to Abraham and believed in types and shadows. The substance of the covenant of grace has always been Christ.
Here it is important to note the phrase “counted to him”, which Paul will later discuss in terms of imputation. Contra Roman Catholicism which sees faith as an infused righteousness (a faith working by love), the Reformed have understood this to mean an alien righteousness, meaning the source of the righteousness comes from outside of us. Calvin notes that the faith of Abraham is the formal cause of our justification and not the efficient cause. The meaning here is not that the act of faith in Abraham is what justifies him but rather that the faith that is given by God is the instrument by which God counts us righteous. By this, all of our works are excluded and all our merit is but filthy rags. The active and passive obedience of Christ – his suffering, death, and obedience to the law – is the meritorious cause. It is what God looks to in fulfilling the requirements by which one may be called righteous.
Understand this. All that you have been given in your salvation is apart from you. You have done nothing to receive the gift of eternal life. There is none of your own obedience in your faith nor any act on your part by which you are reconciled to the Father. It is all of grace. It is all a gift. It is all rooted in Christ and His work.
This is why you can rest in Him, trusting that he has fulfilled all the demands and has given to you all that He is.
Soli Deo Gloria!