Here is another instance of an early teaching on the Eucharist, coming from the Quintisext Council of 692. This is an Eastern Council and obviously some Canons here even highlighted the divergence of practice between East and West, such as the affirmation that married men be admitted to the subdiaconate, diaconate, and presbyteriate as opposed to the growing practice in the Roman Church of a celibate priesthood.
Here is the Canon:
Canon XXVIII.
Since we understand that in several churches grapes are brought to the altar, according to a custom which has long prevailed, and the ministers joined this with the unbloody sacrifice of the oblation, and distributed both to the people at the same time, we decree that no priest shall do this for the future, but shall administer the oblation alone to the people for the quickening of their souls and for the remission of their sins. But with regard to the offering of grapes as first fruits, the priests may bless them apart [from the offering of the oblation] and distribute them to such as seek them as an act of thanksgiving to him who is the Giver of the fruits by which our bodies are increased and fed according to his divine decree. And if any cleric shall violate this decree let him be deposed.
Obviously the issue is what we will call today a Liturgical abuse of mixing grapes with the wine that is offered in oblation (wine that is consecrated for the Eucharist). But we can’t miss the part where the Fathers of the Council themselves affirm that the Eucharist is given to the people for the remission of their sins.
Quickening means life-giving.
Also “When venial sin is remitted, the guilt is removed and as much of the temporal punishment as the person’s dispositions warrant from the grace of God.” - Modern Catholic Dictionary
And from the Council of Trent
Session XXII, Chapter II
THE SACRIFICE OF THE MASS IS PROPITIATORY BOTH FOR THE LIVING AND THE DEAD
And inasmuch as in this divine sacrifice which is celebrated in the mass is contained and immolated in an unbloody manner the same Christ who once offered Himself in a bloody manner on the altar of the cross, the holy council teaches that this is truly propitiatory and has this effect, that if we, contrite and penitent, with sincere heart and upright faith, with fear and reverence, draw nigh to God, [10] For, appeased by this sacrifice, the Lord grants the grace and gift of penitence and pardons even the gravest crimes and sins. For the victim is one and the same, the same now offering by the ministry of priests who then offered Himself on the cross, the manner alone of offering being different. The fruits of that bloody sacrifice, it is well understood, are received most abundantly through this unbloody one, so far is the latter from derogating in any way from the former. Wherefore, according to the tradition of the Apostles,[11] it is rightly offered not only for the sins, punishments, satisfactions and other necessities of the faithful who are living, but also for those departed in Christ but not yet fully purified.
10 Heb. 4:16.
11 Cf. , can. 3, and Sess. XXV, decr. on Purgatory.
As the Church has the power to bind and loosen sins, it may require, as it will, that individual integral confession occur prior to reception of the Eucharist. This condition may change over time. The sin is forgiven if the Church allows it to be forgiven.