The early Christians took the greatest care to conceal the doctrine as well as the celebration of the holy mysteries from pagans and even from catechumens. This was done out of reverence and awe; also as a precaution, to prevent the uninitiated and uninstructed from being present at divine worship, which could have given rise to misconceptions and brought down on them persecution. They were well aware that the teaching of the cross was unto the Jews a stumbling-block, unto the Gentiles foolishness. Accordingly it was never mentioned in the presence of Jews or Gentiles, and even the catechumens who were desirous of being baptized, and who were admitted to Christian instructions, were obliged to leave the church after the first part of the Mass was ended, before the offertory. The doctrine of the Adorable Sacrament of the Altar and of the holy sacrifice of the Mass was not expounded to them until after their Baptism. On this subject St. Cyril of Jerusalem says: "When catechumens are present we do not speak of the holy mysteries in a manner that they can understand; we are often compelled to make use of enigmatical language, which the faithful who are duly instructed will comprehend, but which awaken no suspicions in the mind of the uninstructed." It certainly would not have been necessary to use these precautions had the matter in question been merely a commemorative feast, at which common bread and wine were partaken of.

An Explanation Of The Holy Sacraments
Rev. H. Rolfus, D.D.