It seems to me that this is a very important question, on two fronts. First, it is important to note that Abraham was in fact converted from idolatry. He, like all sons of Adam, was dead in his trespasses and sins and needed to have the righteousness of another granted or imputed to him. He was an idolater, and had no righteousness of his own. We know he was an idolater from the account we are given in Joshua.
“And Joshua said unto all the people, Thus saith the LORD God of Israel, Your fathers dwelt on the other side of the flood in old time, even Terah, the father of Abraham, and the father of Nachor: and they served other gods. And I took your father Abraham from the other side of the flood, and led him throughout all the land of Canaan, and multiplied his seed, and gave him Isaac” (Josh. 24:2-3).
So Abraham was not just an uncircumcised Gentile when God first called to him, he was an idolatrous Gentile. But a second point also needs to be emphasized. Although he clearly needed to be converted, Abraham was not converted in Genesis 15. When did Abraham first respond to the true and living God in genuine faith?
“By faith Abraham, when he was called to go out into a place which he should after receive for an inheritance, obeyed; and he went out, not knowing whither he went. By faith he sojourned in the land of promise, as in a strange country, dwelling in tabernacles with Isaac and Jacob, the heirs with him of the same promise: For he looked for a city which hath foundations, whose builder and maker is God” (Heb. 11: 8-10).
So when Abraham left Ur of the Chaldees, he was responding to the word of God in faith (Gen. 12:1-2). He built several altars to the Lord in Canaan, calling upon the name of the Lord there (Gen. 12:7-8). He is clearly a faithful believer, father of all faithful but uncircumcised Gentiles (Rom. 4:11).
A few chapters later, God reiterates His promises to Abraham, promises that Abraham has already heard and already believed. And it says, well into Abraham’s life of faith, “And he believed in the LORD; and he counted it to him for righteousness” (Gen. 15:6). St. Paul quotes this as a hinge in his argument in Romans 4. In Genesis 17, God reiterates the promise yet again, and grants the sign of circumcision. Abram is renamed Abraham.
Now, here is the question that may make all the participants on all sides of the federal vision discussion unhappy with me, but here goes anyhow. When was Abraham given a new heart? When was he converted? When was he raised out of his condition of Adamic death and brought into a living relationship with God? When was the righteousness of Christ imputed to him in the theological sense? I am not asking whether these things happened, for I affirm that all of them happened. I am asking when.
The answer that exegesis demands is that he was converted when he left Ur of the Chaldees, and he was not converted in Genesis 15. But that means that the language of imputation in Genesis is not as narrow has some have assumed. And on the other side, it appears just as plain to me that what the New Testament teaches about heart regeneration can and must be applied to Abraham in Genesis 12.