It is a common misconception that, for one to become a muzlim, one should circumcise oneself. And/or utter a testimony of faith.
The Kuran (4:119) does not even allow cutting in the ears of cattle, and says it is Shaytan.
Furthermore it says about hypocrites, that they testify in this manner, in its own chapter La Munafiqs.
al-Munafiqun (The Hypocrites) at IslamAwakened.com