It was the Apostolic practice, and the Old Testament practice before that, to require extensive catechetical instruction before allowing one to partake in the feast of the Lord’s Supper (and the Passover).   It is a certain sign of ecclesiastical degeneration when those poorly informed in the chief doctrines of the scriptures are allowed to partake.  And allowing those who knowingly or ignorantly embrace sundry heresies detracts from the unity and health of the church.  How can two walk together unless they be agreed?

The early church respected this principle.  Accordingly, Schaff records in History of the Christian Church, Vol. 2, p. 255 that two to three years of catechism was the norm before adults could partake.   For instance, the council of Elvira alludes to the custom of making it last two years and the civil law fixed it at this (Justinian, Novel. cxliv).  And children were typically catechized several years before partaking some time beginning after the age of twelve.  There had to be an understanding of the chief doctrines through catechetical instruction, an assent to them, and a profession to follow Christ as personal Lord and Savior by seeking to live consistent with the doctrines.

"Catechumen" was the term in the early Church assigned to those undergoing a course of preparation for the purpose of partaking of the Lord’s Supper. The word occurs in Gal. vi, 6: "Let him that is instructed in the word, [ho katechoumenos, is qui catechizatur] communicate to him that instructeth him [to katechounti, ei qui catechizat] in all good things."  Other parts of the verb katicksein occur in I Cor., xiv, 19; Luke, i, 4; Acts, xviii, 24.

As the acceptance of Christianity involved belief in a body of doctrine and the  observance of the Divine law ("teach, make disciples, scholars of them"; "teaching them to observe all things whatever I have commanded you", Matt.,  xxviii, 20), it is clear that instruction must have been given to the adult converts from paganism, as well as children preparing for Christian adulthood.  The church fathers rebuked the heretics for disregarding such catechetical instruction before initiation into communion.  As Tertullian noted: "one does not know which is the catechumen and which the faithful, all alike come [to the mysteries], all hear the same discourses and say the same prayers" (quis catechumenus, quis fidelis incertum est; pariter adeunt, pariter  audiunt, pariter orant), "Catechumens are initiated before they are instructed" (ante sunt perfecti catechumeni quam edocti.--"De Praeser."xli, P.L., II, 56)  As the church degenerated over the centuries, the standards for catechetical instruction before communion were made increasingly lax.  The Protestant Reformation reversed that trend, and thus lifted the requirements for partaking in the Lord’s Supper to their Biblical norms. But most Protestant churches in the following centuries have crept back into error.  There is widespread heresy in modern Protestant churches, and there is little doctrinal unity which should characterize the communion feast.  Any true reformation of the churches must include a reformation of this aspect of church life.