For an adult Xtian to undergo circumcision just to pretend with subterfuge that he's a Jew and then go in to "deep cover" in the Haredi community, shows great "mesirus nefesh" and just shows the strength of their belief!
As the Lubavitcher Rebbe said: zey gleiben in sheker mit'n emes - mir gleiben in emes mit'n sheker - "they believe in lies, but with a true belief - we believe in the truth, but with a false belief!".