Abraham’s calling was a promise with no guidance, explanation or conditions, except moving to Canaan. It was more like a prophecy. Circumcision was not a condition for obtaining the promise, but a sign that marked out the people of promise.
Abram believed the Lord, and he credited it to him as righteousness (Gen 15:6).How was that possible before the cross?
God had already decided that he would forgive anyone who trusted him. Knowing what Jesus would do, he was prepared to forgive their sin and consider them righteous.
The cross was necessary, because the spiritual powers of evil had stolen authority over the earth when Adam and Eve submitted to them. The evil powers refused to let humans free without a ransom being paid. Especially, once the law was given, as it defined transgression and specified the curses on transgression. The spiritual powers of evil demanded the right to implement the curses of the law.
Jesus died on the cross to satisfy the demands of the spiritual powers of evil. His death was the ransom that the law specified for sin. His death redeemed all those who trusted in him from under their authority.
I think Abraham had escaped the spiritual powers of evil by leaving Ur. They thought he was irrelevant, so they just let him go, so they could concentrate their authority near the centre of power. They had not heard God’s call, so they did not understand that Abraham was following God. He just walked out and they let him go free.
Abraham was not perfect, but there was no law yet specifying a curse for transgression, so the spiritual powers of evil had no hold on him. He was dodgy when dealing with Abimelech, but God had accepted his faith and declared him to be righteous. Once God had done that, the spiritual powers of evil could not touch Abraham, because he was under God’s protection. This is why he was able to defeat the kings that took Lot captive.
Abraham’s servant was faithful to his master’s God. I presume his faith was credited to him as righteousness too. He asked God to lead him to the woman who would be a wife for Isaac. God lead him to a good woman. This gave Abraham’s family a great start.