Why is docetism wrong?

For Ignatius docetism is wrong for the following reasons:

(1) The docetics deny that Christ genuinely had a human body.
(2) The docetics deny the suffering, the passion and the resurrection of Christ. As we have seen before, this is particularly a sore point for Ignatius as he saw his own suffering as being linked to the suffering of Christ. Also, the heretics in this way exclude themselves from the salvation Christ achieved.
(3) Docetics deny the Eucharist, as they deny Christ had a real body.
(4) Docetics deny prayer--although Ignatius does not explain this.
(5) Docetics do not care for the widows, the orphans and others who are in need.

There is tension in Ignatius' attitude towards the heretics. On the one hand, he instructs his readers not to associate with them so as not to be contaminated by their teaching. Yet they are to pray for the docetics that they may be saved. In fact, Ignatius refuses to name the heretical teachers, expressing a hope that they might still repent.

Ignatius' Epistle to the Smyrnaeans

As one reads through these epistles by Ignatius, common themes start to emerge. Once again in this letter we have emphasis on the authority of the bishop and the need for submission to him. In fact, it is considered wrong to celebrate the Eucharist without the bishop, or at least someone authorised by him.

However, in this letter the authority of the bishop is set in the context of conflict over the docetic heresy. As I have suggested earlier, prior to a NT canon it is the authority of the bishop which guarantees continuity within the apostolic tradition, hence the need to avoid heretic celebrations of the Eucharist.