When thou commest to this seale of thy peace, the Sacrament, pray that God will give thee that light, that may direct and establish thee, in necessary and fundamental! things; that is, the light of faith to see, that the Body and Bloud of Christ, is applied to thee, in that action; But for the manner, how the Body and Bloud of Christ is there, wait his leisure, if he have not yet manifested that to thee: Grieve not at that, wonder not at that, presse not for that; for hee hath not manifested that, not the way, not the manner of the presence in the Sacrament, to the Church to starve.STC writes: 'O! I have ever felt & for many years thought, that this rem credimus, modum nescimus, is but a poor evasion. It is a seems to me an attempt so to admit an irrational proposition as to have the credit of denying it.' Whalley translates the Latin phrase: 'We believe what is done, but we do not know how it is done'. He notes that 'C. also used it in [ie., the marginalia upon] Jeremy TAYLOR, Polemicall Discourses i.227' (he might have added: he uses it in a marginalium upon Sherlock too). But he cannot locate the Latin: 'the source of the Latin phrase is not traced.'
Well, I traced it. In fact, it is from Dalmatian cleric Marco Antonio Dominis (1560–1624). I came across it, as it were, second hand:
Absurdè dicitur: Rem credimus, modum nescimus. Nam verba materialia non sunt res ipsa, quae creditur. [Adrian van Walenburch and Peter van Walenburch, De articulis fidei necessariis, essentialibus, seu fundamentalibus (1666), 246]'It is absurd to say We believe what is done, but we do not know how it is done; for the words describing material things are not the things themselves, which is what is believed.' This is the Bavarian (Catholic) Van Walenburch brothers attacking the writing of 'Marcus Antonius de Dominis, Archbishop of Spalatro' -- famous in his day for being a Catholic who had come to England and converted to Protestantism. It is Marcus Antonius who says 'rem igitur credimus, modum nescimus' [De republica ecclesiastica 7:12, p.320], which the van Walenburchs here mock. The specific thing that prompts Marcus Antonius' reflection is the story of Christ's descent into hell: we're not sure of the specifics of this voyage, he says, but we believe it nonetheless: 'Non enim certum habemus, eam particulam Symboli Apostolici, qua descensum Christi ad inferos fatetir … rem igitur credimus, modum nescimus.'
Here is an account of the Archbishop's career from The Monthly Review [48 (1805), 333] which Coleridge may have seen:
The most extraordinary character noticed in the biography [An Historical and Descriptive Account of the Royal Hospital and the Royal Military Asylum at Chelsea] is that of the Archbishop of Spalatro, who was admitted a Member of this College by the King's Letters patent in 1622: "Marcus Antonius de Dominis, Archbishop of Spalatro, came over to England in 1616, and professed the protestant religion, asserting that he had discovered various errors in the tenets of the church of Rome, and published his work, De Republica Ecclesiastica: his powers of disputation were strong and acute, his society much courted by the learned and the great, and his sermons attractive and greatly admired. Fuller, who is virulent in his abuse of him, says, that his sole object in coming to England was the attainment of wealth and preferment. King James gave him, soon after his arrival, the deanery of Windsor, the rich living of Illesley, in Berkshire, and made him master of the Savoy. With these, however, he was not contented; but upon the report of the death of Toby Matthew, Archbishop of York, he solicited the king for the vacant archbishoprick; this being refused, he made application for leave to retire to Rome. After much, deliberation, he was ordered to quit the kingdom in twenty days, as he had been found guilty of holding a secret correspondence with the pope, without the king's knowledge. After living some time in poverty and obscurity at Rome, on a small pension allowed him by Pope Gregory XV. he died there in 1625, and his body was afterwards publicly burnt for heresy. Fuller sums up his character with observing—" that he had too much wit and learning to be a cordial papist, and too little honesty and religion to be a sincere protestant."Bonus interesting fact: De Dominis is lampooned in Thomas Middleton's 1624 play A Game at Chess as the 'Fat Bishop of Spalato' who changes faiths whenever it suits him.