In 1277, Robert Kilwardby, archbishop of Canterbury, pronounced at Oxford, the-condemnation of some Aristotelian theses including Thomist theory of unity of forms. In 1284, his successor, John of Peckham ratified his enactment. The aim of this article is to show the reason why these two archbishops issued such prohibitions against St. Thomas, one of the greatest Christian thinkers It is certain that the two archbishops thought that the theory of unity of forms threatened the traditional interpretation on dogmas on the creation and the body of Christ. But they did not ask the sanction of papal court before announcing this prohibition. So, it can be said that on the part of Roman curia, the Pope did not give any suggestion to the archbishops in issuing this condemnation. Therefore, the two archbishops are fully responsible on their action. However, they issued the condemnation not only in accordance with their personal judgement, but they followed the atmosphere of Christian world in which there were still many Augustinians. In those days circles of prelates were totally Augustinians and anti Thomists. These Augustinians were seriously concerned of the new pagan elements in the world of Christian thoughts. The enactment of Kilwardby and Peckham is the reflection of this atmosphere in the Christian world. But it is to be regretted that they failed to grasp, the situation of Thomism which was spreading rapidly in the world of Christian thought and which was adopted by the Dominican order, one of the most important supporters of the papacy, as their official opinions. If they had known better about this situation before issuing their condemnation, they would have been more careful. Their condemnation caused a serious discrepancy among the Christian world which resulted in the disintegration of medieval Christendom.