It looks like St. Ignatius is dealing with a few heresies in this letter: the heresy that Christ did not suffer on the cross and the heresy that there was no bodily resurrection.
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]....
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 did eat and drink with them, as being possessed of flesh, although spiritually He was united to the Father.
He has several instructions for dealing with the heretics, but this section seems to be the most direct:
CHAPTER VII.--LET US STAND ALOOF FROM SUCH HERETICS.
They abstain from the Eucharist and from prayer, because they confess not the Eucharist to be the flesh of our Saviour Jesus Christ, which suffered for our sins, and which the Father, of His goodness, raised up again. Those, therefore, who speak against this gift of God, incur death in the midst of their disputes. But it were better for them to treat it with respect, that they also might rise again. It is fitting, therefore, that ye should keep aloof from such persons, and not to speak of them either in private or in public, but to give heed to the prophets, and above all, to the Gospel, in which the passion [of Christ] has been revealed to us, and the resurrection has been fully proved. But avoid all divisions, as the beginning of evils.
After discussing the heresies, he goes back to one of the themes I sense as common to most of his letters: the need for unity and governance.