Galatians 5:2, 6, 11

Notice, it is I, Paul, who tells you that if you receive circumcision [as a supposed requirement of salvation], Christ will be of no benefit to you [for you will lack the faith in Christ that is necessary for salvation].

For [if we are] in Christ Jesus neither circumcision nor uncircumcision means anything, but only faith activated and expressed and working through love.

But as for me, brothers, if I am still preaching circumcision [as I had done before I met Christ; and as some accuse me of doing now, as necessary for salvation], why am I still being persecuted [by Jews]? In that case the stumbling block of the cross [to unbelieving Jews] has been abolished.