Some mythicists (those who believe there was no historical Jesus) have claimed that the existence of Docetism in the early Church is proof of their claims. A major stretching of the facts is required to come to this conclusion.
In his book The Fathers Know Best, Jimmy Akin defines Docetism this way:
This view held that Jesus did not really have a body, that he was a supernatural being who only appeared to have one. The name comes from the Greek word dokein (“to appear,” “to seem”). The view appears to have existed in the first century, and some passages in the New testament may be meant to refute it (Lk 24:37-43; Jn1:14; 4:1-3; 2 Jn 7). It was definitely present in the early 100s, when St. Ignatius of Antioch wrote against it, and it continued for some time, in conjunction with Gnosticism. (The Fathers Know Best, pg 85)
It should be noted that Docetism, much like Gnosticism, is a catch-all term that can be hard to define. There is no single unified set of beliefs for either of them. The common thread that can be found in Docetism is the belief that Jesus only seemed to be human, but was in fact an immaterial spiritual being.
Some mythicists claim that Docetism in fact proves that there was doubt of a historical Jesus even in the age of the apostles. Rene Salm explains:
We commonly conceive of docetists as those in ancient times who thought Jesus of Nazareth existed, yet who curiously also believed that he did so without a physical body. This doesn’t make sense. The dual understandings of “Jesus” . . . allow us to see the ancient docetists as originally being nonbelievers in the existence of Jesus of Nazareth. They were the first mythicists. (Salm, Mythicists, Docetists, Nazoreans)
Salm’s theory rests on the highly speculative view that the early Christians had a “dual understanding of Jesus.” His claim that Docetism “doesn’t make sense,” is, in fact, quite easy to understand. The Original Catholic Encyclopedia explains:
Docetism is not properly a Christian heresy at all, as it did not arise in the Church from the misunderstanding of a dogma by the faithful, but rather came from without. Gnostics starting from the principle of antagonism between matter and spirit, and making all salvation consist in becoming free from the bondage of matter and returning as pure spirit to the Supreme Spirit, could not possibly accept the sentence, "the Word was made flesh," in a literal sense. In order to borrow from Christianity the doctrine of a Saviour who was Son of the Good God, they were forced to modify the doctrine of the Incarnation.
Docetism is connected to Gnosticism in the sense that both believed matter to be evil and a hindrance to the soul. The Docetists were aware that Jesus actually existed, but in order for them to shoehorn their belief that matter is evil into Christianity, they had to deny that Jesus had a body in the physical sense. Docetism does not deny that a historical Jesus existed. In fact it is evidence to the contrary.
Furthermore, the fact that St. Ignatius of Antioch wrote against it in the early 100s again confirms the historicity of Jesus Christ. Igantius, who died around 110, was a student of the apostle John. As an apostle, John actually spent time with Jesus and must have relayed this to Ignatius. Otherwise, it would be a curious thing that he would argue against Docetism at all. Jesus was a real man, not a phantasm who only appeared to be one. St Ignatius writes:
Now, He suffered all these things for our sakes, that we might be saved. And He suffered truly, even as also He truly raised up Himself, not, as certain unbelievers maintain, that He only seemed to suffer, as they themselves only seem to be [Christians]. And as they believe, so shall it happen unto them, when they shall be divested of their bodies, and be mere evil spirits.
For I know that after His resurrection also He was still possessed of flesh, and I believe that He is so now. When, for instance, He came to those who were with Peter, He said to them, "Lay hold, handle Me, and see that I am not an incorporeal spirit." And immediately they touched Him, and believed, being convinced both by His flesh and spirit. For this cause also they despised death, and were found its conquerors. And after his resurrection He ate and drank with them, as being possessed of flesh, although spiritually He was united to the Father. (The Epistle of Ignatius to the Smyrnaeans, 2-3)
Not a single heresy in the early Church denies the existence of Jesus Christ. None of the Church Fathers deny his existence, and none of the heretics and enemies of the Church do either. Cumulatively, this is all strong evidence for the historicity of Jesus Christ.
If you would like to know more about what the early Christians taught and believed, I suggest picking up a copy of The Fathers Know Best: Your Essential Guide to the Teachings of the Early Christians by Jimmy Akin.