Mizpeh, you are correct. The imputation comes through faith in Christ alone. It was God's promise of the coming Messiah Abraham believed. The testimony of the Father is of Christ. Imputation comes when we accept the testimony God gave of his Son. It was Abraham's faith in the coming Christ which was imputed to him for righteousness.
Gen 15:4-6And, behold, the word of the LORD came unto him, saying, This shall not be thine heir; but he that shall come forth out of thine own bowels shall be thine heir. And he brought him forth abroad, and said, Look now toward heaven, and tell the stars, if thou be able to number them: and he said unto him, So shall thy seed be. And he believed in the LORD; and he counted it to him for righteousness.
1John 5:10-12He that believeth on the Son of God hath the witness in himself: he that believeth not God hath made him a liar; because he believeth not the record that God gave of his Son. And this is the record, that God hath given to us eternal life, and this life is in his Son. He that hath the Son hath life; and he that hath not the Son of God hath not life.
We have faith in God by accepting his word/testimony to be true.
Prax, do you believe ALL Orthodox Jews are justified because of their faith in YHWH?