How could Abraham be declared righteous before the death of Christ?

So, also, Abraham’s faith, combined with works, was “counted [reckoned, credited, or attributed] to him as righteousness.” (Ro 4:20-22) This, of course, does not mean that he and other faithful men of pre-Christian times were perfect or free from sin; yet, by virtue of their exercise of faith in God’s promise concerning the “seed” and because they were striving to follow God’s commands, they were not classed as unrighteous with no standing before God, like the rest of the world of mankind. (Ge 3:15; Ps 119:2, 3) Jehovah lovingly accounted them guiltless, when compared with the world of mankind alienated from God. (Ps 32:1, 2; Eph 2:12) Thus, God could, by reason of their faith, have dealings with such imperfect men and bless them, doing so while still remaining true to his own perfect standards of justice. (Ps 36:10) However, such ones recognized their need for redemption from sin and were awaiting God’s due time to provide it.​—Ps 49:7-9; Heb 9:26.

Indicating the scope of Abraham’s justification, the disciple James wrote: “The scripture was fulfilled which says: ‘Abraham put faith in Jehovah, and it was counted to him as righteousness,’ and he came to be called ‘Jehovah’s friend.’” (Jas 2:20-23) So, because of his faith, Abraham was declared righteous as a friend of God, not as a son of God because of being “born again” with heavenly life in view. (Joh 3:3)