Genesis 17

13 years time pass between Genesis 16 where the Angel of the LORD appeared to Hagar, and Genesis 17:1 where the LORD appeared to Abram.  It's certainly important to take note of the amounts of time that pass between the encounters with God that are recorded. And though Abram has walked by faith for 24 years, God still commands Abram to walk before Him and be blameless. 

God will in an orderly fashion give great detail in affirming the covenant that He is making between Abraham and his descendants. We will see God say "as for Me" in verse 4, "as for you" in verse 9,  "as for Sarai" in verse 15, and "as for Ishmael" in verse 20. God will also establish a sign of the covenant in circumcision. We take note of Abraham's immediate obedience, and the fact that he had a good enough testimony in his household that all of the men of his house were willing to obey what God commanded even though it might have been considered quite inconvenient.

As we get into the New Testament, we will see circumcision is now fulfilled spiritually in the heart of the believer by the "circumcision of Christ"  as Paul will refer to it in Col 2:11.