Faith was reckoned to Abraham for justification, as we read Genesis 15:6, (see the note on Genesis 15:6); but circumcision was not instituted till about fourteen or fifteen years after, Genesis 17:1, etc.; for faith was reckoned to Abraham for righteousness or justification at least one year before Ishmael was born; compare Genesis 15, and 16. At Ishmael’s birth he was eighty-six years of age, Genesis 16:16; and, at the institution of circumcision, Ishmael was thirteen, and Abraham ninety-nine years old. Dr. Adam Clark
"How then was it credited? While he was circumcised, or uncircumcised? Not while circumcised, but while uncircumcised;" (NASB)
"How was it then reckoned? when he was in circumcision, or in uncircumcision? Not in circumcision, but in uncircumcision." (KJV)
"But how did this happen? Was he counted as righteous only after he was circumcised, or was it before he was circumcised? Clearly, God accepted Abraham before he was circumcised!" (NLT)
"How then was it credited to him? Was he circumcised at the time, or not? No, he was not circumcised but uncircumcised!" (NET)
I suppose the argument here portrayed is similar to the ordinance of Christian baptism. One must consider whether one is credited before or after baptism/circumcision. But the Apostle has already dispatched this thought in Chapter Two:
For a person is not a Jew who is one outwardly, nor is circumcision something that is outward in the flesh, but someone is a Jew who is one inwardly, and circumcision is of the heart by the Spirit and not by the written code. This person's praise is not from people but from God.
(Romans 2:28-29 NET.)
As to the time line, the distance between Genesis 15:6 and Genesis 17:1 is about 13 - 15 years. As to the possible objection by the Hebrew, there is none because Abraham's faith was expressed long before the rite of circumcision. There are a great many lessons that could be provoked from this passage. However, the one that comes to mind is that we must not rest upon the physical act of baptism (nor any other) but cling to the faith that has set us free in Christ Jesus. We are creatures of now and cannot hold to past successes or circumstances. Who we are today is who we are! That is why Christ has created a new day and called it today whereby we may enter into His Sabbath rest. If we are hard hearted or arrogant we will fail and lose out on a daily blessing. If we recognize that we are still living in this fallen fleshly container and have need of the renewing of our minds by the Holy Spirit in order to face the challenges of each and every minute, then we will be able to enter this rest.
Faith was reckoned to Abraham for justification, as we read Genesis 15:6, (see the note on Genesis 15:6); but circumcision was not instituted till about fourteen or fifteen years after, Genesis 17:1, etc.; for faith was reckoned to Abraham for righteousness or justification at least one year before Ishmael was born; compare Genesis 15, and 16. At Ishmael’s birth he was eighty-six years of age, Genesis 16:16; and, at the institution of circumcision, Ishmael was thirteen, and Abraham ninety-nine years old. Dr. Adam Clark
Abraham’s age when he was declared righteous (Gen. 15:6) is not stated. But later when Hagar bore him Ishmael, he was 86 (Genesis 16:16). After that, God instructed Abraham to perform the rite of circumcision on all his male descendants as a sign of God’s covenant with him; this was done when Abraham was 99 (Genesis 17:24). Therefore the circumcision of Abraham followed his justification by faith by more than 13 years. Walvoord, J. F., Zuck, R. B., & Dallas Theological Seminary. (1983-c1985). The Bible knowledge commentary : An exposition of the scriptures (Ro 4:9). Wheaton, IL: Victor Books.
And Abram believed the LORD, and the LORD counted him as righteous because of his faith. (Genesis 15:6 NLT)
When Abram was ninety-nine years old, the LORD appeared to him and said, "I am El-Shaddai—'God Almighty.' Serve Me faithfully and live a blameless life. (Genesis 17:1 NLT)
Abraham was ninety-nine years old when he was circumcised, and Ishmael, his son, was thirteen.
(Genesis 17:24-25 NLT)