In him also you were circumcised with a spiritual circumcision
The Jews would exclude or at the very least belittle the Gentiles for their lack of the identifying mark of the chosen people.
“From Paul’s radical perspective, circumcision in the flesh had become little more than a tribal brand. In Romans 2:25-29 , he argues that circumcision, the ground of Jewish confidence, is a meaningless sign unless it signifies a cleansed heart; and he goes on to argue that the cleansed heart can only come from Christ.”
The same goes for baptism.
It is never faith in baptism, but rather faith in Christ who in baptism marks us as his own.