The interpretation timeline

The Fourth Lateran Council

Issued the creed Firmiter credimus, which states that the body and blood are truly contained under the species of bread and wine, transubstantiated.

1215 Rome, the Lateran
Convoked under
Pope Innocent III
Present
the Patriarchs of Constantinople and Jerusalem, 71 archbishops, 412 bishops, 800 abbots, the Primate of the Maronites, and Saint Dominic
What it did
issued an enlarged creed (Firmiter credimus) against the Albigenses; condemned the Trinitarian errors of Abbot Joachim; published 70 reformatory decrees
How the 1913 encyclopedia rates it
“the most important council of the Middle Ages, and it marks the culminating point of ecclesiastical life and papal power”

These facts as given by the Catholic Encyclopedia (1913), "Councils" PD

Scholastic

c. 1100 – 1500
1215
Event
The Fourth Lateran Council

Lateran IV issues the creed Firmiter credimus, the first conciliar text to use the word transubstantiated of the Eucharist.

1215
A.D.
“We firmly believe and simply confess that there is one only true God, eternal, immeasurable and unchangeable, incomprehensible, almighty and ineffable, Father and Son and Holy Spirit: three Persons indeed, but one essence, substance or nature altogether simple. The Father from none, the Son from the Father alone, and the Holy Spirit equally from both: without beginning, always and without end; the Father begetting, the Son being born, and the Holy Spirit proceeding; consubstantial and coequal and coalmighty and coeternal; one principle of all things; creator of all things visible and invisible, spiritual and corporeal; who by his almighty power from the beginning of time made together out of nothing both orders of creature, the spiritual and the corporeal, that is the angelic and the earthly, and thereafter the human, as it were common to both, composed of spirit and body. For the devil and the other demons were indeed created by God good in their nature, but they made themselves evil of their own doing. Man however sinned at the suggestion of the devil. This Holy Trinity, undivided according to its common essence and distinct according to the properties of the Persons, gave to the human race the doctrine of salvation, first through Moses and the holy Prophets and his other servants, according to the most orderly disposition of the times. And at the last the only-begotten Son of God, Jesus Christ, made flesh by the whole Trinity in common, conceived of Mary ever Virgin by the co-operation of the Holy Spirit, made true man, composed of a rational soul and human flesh, one Person in two natures, showed more plainly the way of life. Who, although according to his divinity he is immortal and impassible, the same according to his humanity was made passible and mortal; who also, for the salvation of the human race, having suffered and died upon the wood of the cross, descended to the realms below, rose again from the dead and ascended into heaven — but he descended in the soul and rose again in the flesh, and ascended alike in both — to come at the end of the world, to judge the living and the dead, and to render to each according to his works, to the reprobate as to the elect: who shall all rise again with their own bodies, which they now bear, that they may receive according to their works, whether these have been good or evil; the one everlasting punishment with the devil, and the other everlasting glory with Christ. There is moreover one universal Church of the faithful, outside which no one at all is saved; in which Jesus Christ himself is both priest and sacrifice, whose body and blood are truly contained in the sacrament of the altar under the species of bread and wine, the bread having been transubstantiated into the body and the wine into the blood by divine power, so that, to accomplish the mystery of unity, we ourselves may receive of what is his what he himself received of what is ours. And this sacrament no one at all can confect but a priest who has been rightly ordained according to the keys of the Church, which Jesus Christ himself granted to the Apostles and their successors. But the sacrament of baptism — which is consecrated in water at the invocation of God and the undivided Trinity, namely of the Father and of the Son and of the Holy Spirit — rightly conferred by anyone in the form of the Church, profits unto salvation both little ones and adults. And if anyone, after receiving baptism, has fallen into sin, he can always be restored through true penance. Moreover not only virgins and the continent, but also the married, pleasing God by right faith and good works, deserve to come to eternal blessedness.”
Firmiter credimus et simpliciter confitemur, quod unus solus est verus Deus, aeternus, immensus et incommutabilis, incomprehensibilis, omnipotens et ineffabilis, Pater et Filius et Spiritus Sanctus: tres quidem personae, sed una essentia, substantia seu natura simplex omnino: Pater a nullo, Filius a Patre solo, ac Spiritus Sanctus pariter ab utroque: absque initio, semper ac sine fine: Pater generans, Filius nascens, et Spiritus Sanctus procedens consubstantiales et coaequales et co omnipotentes et coaeterni: unum universorum principium: creator omnium visibilium et invisibilium, spiritualium et corporalium: qui sua omnipotenti virtute simul ab initio temporis utramque de nihilo condidit creaturam, spiritualem et corporalem, angelicam videlicet et mundanam: ac deinde humanam, quasi communem ex spiritu et corpore constitutam. Diabolus enim et alii daemones a Deo quidem natura creati sunt boni, sed ipsi per se facti sunt mali. Homo vero diaboli suggestione peccavit. Haec Sancta Trinitas, secundum communem essentiam individua, et secundum personales proprietates discreta, primo per Moysen et sanctos Prophetas, aliosque famulos suos, iuxta ordinatissimam dispositionem temporum doctrinam humano generi tribuit salutarem. Et tandem unigenitus Dei Filius lesus Christus, a tota Trinitate communiter incarnatus, ex Maria semper Virgine Spiritus Sancti cooperatione conceptus, verus homo factus, ex anima rationali et humana carne compositus, una in duabus naturis persona, viam vitae manifestius demonstravit. Qui cum secundum divinitatem sit immortalis et impassibilis, idem ipse secundum humanitatem factus est passibilis et mortalis qui etiam pro salute humani generis in ligno crucis passus et mortuus, descendit ad infernos, resurrexit a mortuis et ascendit in coelum: sed descendit in anima, et resurrexit in carne: ascenditque pariter in utroque: venturus in fine saeculi, iudicaturus vivos et mortuos, et redditurus singulis secundum opera sua, tam reprobis quam electis qui omnes cum suis propriis resurgent corporibus, quae nunc gestant, ut recipiant secundum opera sua, sive bona fuerint sive mala: illi cum diabolo poenam perpetuam, et isti cum Christo gloriam sempiternam. Una vero est fidelium universalis Ecclesia, extra quam nullus omnino salvatur, in qua idem ipse sacerdos est sacrificium lesus Christus, cuius corpus et sanguis in sacramento altaris sub speciebus panis et vini veraciter continentur, transsubstantiatis pane in corpus et vino in sanguinem potestate divina: ut ad perficiendum mysterium unitatis accipiamus ipsi de suo, quod accepit ipse de nostro. Et hoc utique sacramentum nemo potest conficere, nisi sacerdos, qui rite fuerit ordinatus, secundum claves Ecclesiae, quas ipse concessit Apostolis eorumque successoribus lesus Christus. Sacramentum vero baptismi (quod ad Dei invocationem et individuae Trinitatis, videlicet Patris, et Filii, et Spiritus Sancti, consecratur in aqua) tam parvulis, quam adultis in forma Ecclesiae a quocunque rite collatum proficit ad salutem. Et si post susceptionem baptismi quisquam prolapsus fuerit in peccatum, per veram potest semper poenitentiam reparari. Non solum autem virgines et continentes, verum etiam coniugati, per rectam fidem et operationem bonam placentes Deo, ad aeternam merentur beatitudinem pervenire.
Source
Modern · 1953 →

The in-app commentary runs from the Fathers to the early-modern record, then stops — that's where the public-domain sources end, not where the reading does. For the modern reading, follow the sources directly.