God gave Abraham the covenant of circumcision:
“This is My covenant, which you shall keep, between Me and you and your descendants after you: every male among you shall be circumcised. And you shall be circumcised in the flesh of your foreskin, and it shall be the sign of the covenant between Me and you. And every male among you who is eight days old shall be circumcised throughout your generations, a slave who is born in the house or who is bought with money from any foreigner, who is not of your descendants. A servant who is born in your house or who is bought with your money shall surely be circumcised; thus shall My covenant be in your flesh for an everlasting covenant."
The covenant between God and Abraham and his decedents is eighth day circumcision.
And he gave him the covenant of circumcision: and so Abraham begat Isaac, and circumcised him the eighth day; and Isaac [circumcised] Jacob [on the eighth day]; and Jacob [circumcised] the twelve patriarchs [on the eighth day]. Acts 7:8
In addition to the covenant, God also instructed Abraham to circumcise his adult males and his foreign slaves bought with silver. This is consistent with the passage in Exodus 12. It is not a command for Gentile believers to become circumcised as adults.