Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Canons of the Councils of Ancyra, Gangra, Neocæsarea, Antioch and Laodicea, which Canons were Accepted and Received by the Ecumenical Synods.: Canon XII

Canon XII.

It is decreed that they who have offered sacrifice before their baptism, and were afterwards baptized, may be promoted to orders, inasmuch as they have been cleansed.

Notes.

Ancient Epitome of Canon XII.

Whoso has sacrificed before his baptism, after it shall be guiltless.

Hefele.

This canon does not speak generally of all those who sacrificed before baptism; for if a heathen sacrificed before having embraced Christianity, he certainly could not be reproached for it after his admission.  It was quite a different case with a catechumen, who had already declared for Christianity, but who, during the persecution had lost courage, and sacrificed.  In this case it might be asked whether he could still be admitted to the priesthood.  The Council decided that a baptized catechumen could afterwards be promoted to holy orders.