The Lord's promise of presence belongs to the unanimous, not to schismatics
Cyprian is answering schismatics who misuse the Lord's saying about two or three gathered in his name, showing that the promise presupposes unity with the Church.
Nor should certain people deceive themselves with a false interpretation of what the Lord said: "Wherever two or three are gathered in my name, I am with them." Corrupters of the Gospel and false interpreters put the last words forward and pass over what comes before, remembering part and slyly suppressing part. Just as they themselves have been cut off from the Church, so they split the meaning of a single passage. For when the Lord was urging unanimity and peace upon his disciples, he said: "I tell you, if two of you agree on earth about anything you ask, it will be granted to you by my Father who is in heaven. For wherever two or three are gathered in my name, I am with them" (Matthew 18:19-20). He was showing that much is given not to the number of those praying but to their unanimity. "If two of you," he says, "agree on earth": he put unanimity first and set the harmony of peace before all else. He taught faithfully and firmly that we should agree together. But how can someone agree with another person when he does not agree with the body of the Church itself and with the whole brotherhood? How can two or three be gathered in the name of Christ when it is clear that they have been separated from Christ and from his Gospel? For we did not withdraw from them; they withdrew from us. And since heresies and schisms were born afterward, while they set up various conventicles for themselves, they abandoned the head and source of truth. The Lord, however, speaks about his Church and speaks to those who are in the Church, so that if they are in harmony, if according to what he commanded and urged two or three, however few, pray with one mind, then two or three, however few they are, can obtain from God's majesty what they ask. "Wherever two or three are," he says, "I am with them"—that is, with the simple and the peaceful, with those who fear God and keep God's commandments. He said he is present with these two or three, however few, just as he was with the three young men in the fiery furnace, and because they remained simple toward God and unanimous among themselves, he refreshed them in the midst of the surrounding flames with the spirit of dew (Daniel 3:49-50). So too he was present with the two apostles shut up in prison, because they were simple, because they were unanimous; he himself loosed the prison bars and placed them again in the forum, so that they might hand on to the crowd the word they were faithfully preaching (Acts 5:17-21). Therefore when he lays down in his commands and says, "Where there are two or three, I am with them," he does not divide people from the Church—he who established and made the Church. Rather, reproaching discord to the faithless and commending peace to the faithful by his own voice, he shows that he is more present with two or three praying in unanimity than with a great many who are divided, and that more can be obtained by the harmonious prayer of a few than by the discordant prayer of many.
Latin
Nec se quidam uana interpretatione decipiant, quod dixerit Dominus: ubicumque fuerint duo aut tres collecti in nomine meo, ego cum eis sum. corruptores euangelii adque interpretes falsi extrema ponunt et superiora praetereunt, partis memores et partem subdole conprimentes: ut ipsi ab ecclesia scissi sunt, ita capituli unius sententiam scindunt. Dominus enim cum discipulis suis unanimitatem suaderet et pacem: dico, inquit, nobis quoniam si duobus ex uobis conuenerit in terra, de omni re, quamcumque petieritis, continget nobis a patre meo qui in caelis est. ubicumque enim fuerint duo aut tres collecti in nomine meo, ego cum eis sum, ostendens non multitudini sed unanimitati deprecantium plurimum tribui. siduobus, inquit, ex uobis conuenerit in terra: unanimitatem prius posuit, concordiam pacis ante praemisit: ut conueniat nobis, fideliter et firmiter docuit. quomodo autem potest ei cum aliquo conuenire, cui cum corpore ipsius ecclesiae et cum uniuersa fraternitate non conuenit? quomodo possunt duo aut tres in nomine Christi colligi quos constet a Christo et ab eius euangelio separari ? non enim nos ab illis, sed illi a nobis recesserunt et cum haereses et schismata postmodum nata sint, dum conuenticula sibi diuersa constituunt, ueritatis caput adque originem reliquerunt. Dominus autem de ecclesia sua loquitur et ad hos qui sunt in ecclesia loquitur, ut si ipsi concordes fuerint, si secundum quod mandauit et monuit duo aut tres licet collecti unanimiter orauerint, duo aut tres licet sint, inpetrare possint de Dei maiestate quod postulant. ubicumque fuerint duo aut tres, ego, inquit, cum eis sum, cum simplicibus scilicet adque pacatis, cum Deum timentibus et Dei praecepta seruantibus. cum his duobus uel tribus licet esse se dixit, quomodo et cum tribus pueris in camino ignis fuit et quia in Deum simplices adque inter se unanimes permanebant, flammis ambientibus medios spiritu roris animauit, quomodo apostolis duobus in custodia. clausis, quia simplices, quia unanimes erant, ipse adfuit, ipse resolutis carceris claustris, ut uerbum quod fideliter praedicabant multitudini traderent, ad forum rursus inposuit. quando ergo in praeceptis suis ponit et dicit: ubi fuerint duo aut tres, ego cum eis sum, non homines ab ecclesia diuidit qui instituit et fecit ecclesiam, sed exprobrans discordiam perfidis et fidelibus pacem sua uoce commendans ostendit magis esse se cum duobus aut tribus unanimiter orantibus quam cum dissidentibus plurimis plusque inpetrari posse paucorum concordi prece quam discordiosa oratione multorum.