God appears to Abram and institutes the sign of circumcision as a confirmation of His covenant and He changes the names of Abram and Sarai to Abraham and Sarah.  When there is a change in name, it signifies a change in character. This circumcision is a foreshadowing of circumcision of the heart.

Philippians 3:3
For we are the circumcision who worship God in the spirit and rejoice in Christ Jesus and have no confidence in the flesh;