AlbertDerGrosse:
None of them can demonstrate that even a single early Christian professed a unified, coherent set of Mormon doctrines.
The Church of Jesus Christ plainly teaches that some divine truths will not be revealed until this last gospel dispensation. It would be a mistake to assume every last Latter-day Saint (non-Orthodox Christian) doctrine must be found in an ancient text somewhere in order to support the Latter-day Saint claim of general apostasy. D&C 128:18 states:
…which dispensation is now beginning to usher in, that a whole and complete and perfect union, and welding together of dispensations, and keys, and powers, and glories should take place, and be revealed from the days of Adam even to the present time. And not only this, but those things which never have been revealed from the foundation of the world, but have been kept hid from the wise and prudent, shall be revealed unto babes and sucklings in this, the dispensation of the fulness of times.
For a more thorough explanation see:
Scriptural Mormonism: Notes on D&C 128:18 being a "controlling verse" for LDS Exegesis
Of course, Catholics cannot demonstrate that even a single early Christian professed a unified, coherent
set of Catholic doctrines either.
John Henry Newman said it best. (Just a polite warning this and the two that follow are regurgitated quotes.)
If we limit our view of the teaching of the Fathers by what they expressly state, St. Ignatius may be considered as a Patripassian, St. Justin arianizes. and St. Hippolytus is a Photinian … Tertullian is heterodox on the doctrine of our Lord’s divinity … Origen is. at the very least, suspected, and must be defended and explained rather than cited as a witness of orthodoxy; and Eusebius was a Semi-Arian. (Newman, Essay, 43)
Regarding Ignatius, Fortman says:
Thus although there is nothing remotely resembling a doctrine of the Trinity in Ignatius , the triadic pattern of thought is there, and two of its members, the Father and Jesus Christ, are clearly and often designated as God. (Edmund J. Fortman, The Triune God: A Historical Study of the Doctrine of the Trinity (Philadelphia: Westminster Press, 1972), 40
Regarding Justin Martyr, Fortman says:
On several occasions Justin coordinates the three persons, sometimes citing formulas derived from baptism and the eucharist, sometimes echoing official catechetical teaching. He worshipped the Father as Supreme in the universe; he worshipped the Logos or Son as divine but in the second place ; he worshipped the Holy Spirit in the third place. But he has no real doctrine of the Trinity , for he says nothing of the relations of the three to one another and to the Godhead . (Edmund J. Fortman, The Triune God: A Historical Study of the Doctrine of the Trinity (Philadelphia: Westminster Press, 1972), 47