Against Christians Converting to the Jewish Rite (1267 AD)
Clement, bishop, servant of the servants of God, to his beloved sons, the brothers of the Order of Preachers and of the Order of Minors, inquisitors of heretical depravity appointed by the authority of the Apostolic See, or to be appointed in the future, greeting and apostolic benediction.
With troubled heart we have heard and narrate that very many reprobate Christians, denying the truth of the Catholic faith, have transferred themselves in a damnable manner to the rite of the Jews. Which is known to be all the more reprobate inasmuch as from this the most holy name of Christ is more securely blasphemed by a certain familiar hostility.
Since moreover this damnable pestilence, which, as we have received, grows exceedingly not without the subversion of the aforesaid faith, ought to be opposed with suitable and swift remedies, we command your whole body by apostolic writings that, within the boundaries designated to you for inquiring against heretics by the authority of the Apostolic See, concerning the aforesaid matters, having diligently and faithfully inquired into the truth both through Christians and also through Jews, you proceed against Christians whom you shall have found to have committed such things, as against heretics.
Moreover, punish the Jews who have hitherto induced, or whom you shall find henceforth inducing, Christians of either sex to their execrable rite, with due penalty. Restraining by ecclesiastical censure, with appeal set aside, those who offer opposition. Invoking to this end, if it shall be necessary, the aid of the secular arm.
Given at Viterbo, on the seventh day before the Kalends of August, in the third year of our pontificate.
Source. Bullarium Romanum, Vol. III – Translated by Claude.AI. Clement IV, Contra Christianos ad Iudaeorum Ritum Transeuntes. Bullarium Diplomatum et Privilegiorum Sanctorum Romanorum Pontificum, Taurinensis Editio, Vol. III, Bull XXIV. Turin, 1858.