Laodicea - Canon 7

Persons converted from heresies, that is, of the Novatians, Photinians, and Quartodecimans, whether they were catechumens or communicants among them, shall not be received until they shall have anathematized every heresy, and particularly that in which they were held; and afterwards those who among them were called communicants, having thoroughly learned the symbols of the faith, and having been anointed with the holy chrism, shall so communicate in the holy Mysteries.

Three groups of heretics are specifically mentioned requiring a certain amount of background to understand this canon.

Quartodecimans –	This group followed the practice of observing Easter beginning on the eve of Nisan 14 in honor of the timing of Christ's crucifixion as given in John's gospel (John 19:14, 31, 42) and coinciding with the Passover as based on God's original instruction (Exod 12:18; Lev 23:5; Num 9:5).  The remainder of the corporate church had subscribed to observance on Sunday in honor of the resurrection.  Later this dispute would be over which Sunday was the proper date for the observance.
Novatians –	Novatian was priest during the persecutions from Decius in the third century during which some Christians turned from the faith.  After the persecutions eased and those lapsed desired to return to the church, Cornelius, overseer of Rome, was quick to allow their return, while Novatian took an austere position arguing that those who had fallen should not be so allowed and had himself ordained as a rival of Cornelius in order to enforce his position.  Cyprian attempted to intervene but was unable.  Novation was eventually declared a heretic by the church for his divisiveness.  The sect continued with their austere application of scripture into the seventh century.
Photinians –	Photinus was a fourth century heretic serving as deacon in Ancyra under Marcellus and later became overseer of Sirmium.  He was noted for denying Jesus' incarnation (i.e., God putting on human flesh).

Only the last of these would be considered heresy today, yet in the fourth century, these battles were quite serious.

The only workable solution to receive a former adherent of heresy was to require the prospective person to anathematize every point of the old teaching and cleave to the truth of scripture.  After proper discipling and anointing, that one was allowed fellowship with the church.