What makes a Christian is not one's external works, but one's conscience

"Dr. Karlstadt has fallen from the kingdom of Christ and has suffered shipwreck with respect to faith. Therefore he wants to get us out of the kingdom right into works and simply make Galatians of us also. For take note, dear reader, what gross blindness it is to fight as he does. 'If anyone circumcise himself, should he not in all fairness be called a Jew? Thus, whoever elevates is rightly called one who brings a sacrifice, etc.' You poor, miserable spirit, where on earth have you read that he is rightly called a Jew who circumcises himself? Did not Paul circumcise Timothy, when he was already baptized and a Christian? (Acts 16[:3]). Does not Paul declare circumcision a matter of free choice (1 Cor. 7[:19]), 'Neither circumcision counts for anything nor uncircumcision,' that is, one may circumcise himself or not have a foreskin or not. And this spirit pits his judgment blithely and boldly against that of Paul, saying it is not a matter of free choice, but makes one a Jew. he ought rather say that whoever circumcises himself as though he were compelled to do so by law and for conscience's sake, he is rightly a Jew. For circumcision does not make a Jew, since one does find those who due to illness or on account of an infection must be circumcised. Should they therefore be called Jews?
He, however, is a Jew who, compelled in his conscience by law, feels he must be circumcised. This Jewish disposition and conscience makes one a Jew, even if he never externally circumcised himself or could circumcised himself. The foreskin thus makes no one a Jew. But if he thinks in his conscience, he must have a foreskin, this one is a gentile, even if he permitted himself to be circumcised a thousand times externally. Similarly, since he thinks it necessary to have the foreskin and to condemn circumcision without leaving it free choice as Christ would have it, Dr. Karlstadt actually is a gentile and has lost Christ. Here one sees clearly how this man is completely swallowed up in works and drowned in external appearance, so that he is not able to give one single right judgment in spiritual matters of conscience. For it is impossible that a spark of Christian understanding should still be found in him, since he holds that an external work makes a Jew or Christian, gentile or Turk, and does not judge according to the conscience, but according to semblance and appearance, which even reasonable people do not do." -Martin Luther, Against the Heavenly Prophets, Pg. 217-218