I know from past research, as I’ve learned (and, in the past, believed in) just about every heresy that was preached up until the seventh ecumenical council before I became an orthodox Christian, before I knew there was an actual “orthodoxy” compared to “heresy” (as I have heard my entire life, in Islam and in America, that “heresy came before orthodoxy” and that “primitive Christianity” was “very diverse”, such as that preached by Elaine Pagels, Bart Ehrman, Ahmed Deedat, John Crossan, etc.).
Starting with, as I lost my original faith, pantheism of a sort, after the fashion of Spinoza, then Deism of a sort, after the fashion of Newton (to which I returned, generally in the form of a kind of Deist Arianism, Adoptionism, or Sabellianism, generally the latter, with the Moral Influence Theory of atonement, several times on my journey). Arianism and Sabellianism have a very logical method to them, and are appealing to a rationalist mind like my own, especially when much of the liberal history would claim that Arianism was a majority religion in “early Christianity”, because they Jews never would have worshiped Jesus (not to mention the NT documents or Pliny’s epistle to Trajan), that the NT is completely unreliable and late and compiled by the “proto-orthodox” who “won” some sort of war of ideas in the 4th-5th centuries, and the Jesus Seminar, which would say that not one instance of Jesus’ claim to divinity in the NT is an original “Jesus tradition”. Adoptionism, similarly.
Docetism and Iconoclasm made Christianity seem less alien to my sensibilities, since it is claimed in the Koran that Jesus was not crucified, but Judas in his place, made to appear as Jesus, and, as everyone knows, Islamic art is the most aniconic in the world. When I came closer to orthodoxy, the Hypostatic Union, like the Trinity, seemed to me to be illogical, overly-complicated and confusing; so I bounced from monophysitism to Nestorianism (I’m surprised I didn’t make a stop at Eutychianism, but it was no more logical to my mind than the orthodox doctrine; add adoptionism in here too, if you wish, as an additional Christological heresy) before coming to the orthodox doctrine. Pseudo-Gnosticism (in all Docetic, Adoptionist, and completely Esoteric forms) seemed strong and reasonable, due to its intimate relation with the gods of the Philosophers, Platonism, Plotinism, Neo-Platonism, etc.; I even flirted with Mormonism for a bit, but try as I might, I couldn’t make myself believe their doctrines, no matter what their other-worldly promises, and eventually realized that the call of Mormonism was the call of the Serpent: to eat of this fruit, and become as Elohim is.
Note, as I was learning the history, creeds and heresies of Christianity, I went through all of those within likely a six-week period. If I have one strength, it is that of Anthony Flew: “to follow where the argument does lead”.