The custom of the fermentum was first practiced as early as 120 AD. A particle of the Eucharistic bread was carried by a minister of the Church from the bishop of one diocese to the bishop of another diocese. The receiving bishop would then consume the species at his next celebration of the Eucharist as a sign of the communion between the churches. The term fermentum was probably a reference to the Eucharist as the leaven of the Christian life, and as the instrument by which Christians spread throughout the world were united in the one Body of Christ as a leaven to the world.
In the 2nd century, popes sent the Eucharist to other bishops as a pledge of unity of faith, this being the origin of the expression to be in communion with each other, and such communion already considered essential to Christianity in the 2nd-century writings of St. Ignatius of Antioch and St. Irenaeus. On occasion, bishops also sent out fermenti to their priests.
John. D. Zizioulas (the Metropolitan of Pergamon) in his doctoral thesis (University of Athens, 1964) takes the position that by the mid-3rd century the Bishops were exercising fermentum with the (local to the Metropolitan) parishes that did have a presiding Bishop in order to communicate/retain the unity of the Church under the Bishop. While this was due to the growth of the Christian church it is also due to extensive persecution of the Church, especially aimed at the Bishops.
At papal masses around 700 AD Klausen states: "Saying the salutation 'Pax Domini sit semper vobiscum' – [the peace of the Lord be always with you] – the Pope immersed the particle which came from the previous day's mass in the chalice and at the same time broke off a piece of bread which he had consecrated, and which was to serve as a fermentum at the next service"
- Encyclopedia.Com: Our Sunday Visitor's Catholic Encyclopedia, pg. 437 The rite of committing consists of placing a small particle of the consecrated Bread into the chalice. This custom derives from the fermentum practice (literally, "leaven"). The fermentum was originally part of the Eucharistic Bread consecrated by the Pope in Rome (as attested in the letter of Pope St. Innocent 1 in A.D. 416 to Bishop Decentius of Gubbio) and then sent to the various tituli (parish churches) in the city to denote the unity of the one Catholic community despite their having to celebrate separate Eucharists.
- Encyclopedia.Com: Our Sunday Visitor's Catholic Encyclopedia, pg. 255 The origins of this rite [liturgical commixture] are to be found in fermentum, during which a piece of the consecrated Bread was broken off and sent to be part of another Eucharistic celebration to show the essential unity of the Church in the Eucharistic Sacrifice. When this was no longer done, the piece was dropped into the chalice and medieval allegorical explanations were developed to explain the practice.
- Eusebius of Caesarea's Church History, Book 5, Chapter 24, Paragraph 17 But though matters were in this shape, they communed together, and Anicetus conceded the administration of the eucharist in the church to Polycarp, manifestly as a mark of respect. And they parted from each other in peace, both those who observed, and those who did not, maintaining the peace of the whole church.
- Klauser, Theodor (1979). A short History of the Western Liturgy. Oxford University press. pp. 66–67. ISBN 0-19-213223-7.