The year is 743 AD. Some moslems came to this holy man and asked him a question about religion. He said to these moslems, who probably started out as Christian of the East, that anyone who did not embrace Catholicism is damned. He was speaking to folks who had apostasized from faith in Jesus Christ to follow the the child-molesting false prophet, mohammed.
This is in the mid-8th century, after a hundred years of moslem slaughter and conquest of Christian lands and people. He is telling these moslems that in abandoning Christian faith, they have embraced damnation, that they have no excuse for their crimes, that there is no justice in the century of rape and slaughter and slavery in which they have joined.
At this time, the Church is still (mostly) unified (most days). Thus, when the saint talks about the Catholic Church, he is talking about all Christianity, the Christianity of Rome, the Christianity of Constantinople, of Jerusalem, of Alexandria, of Antioch, and, yes, of Damascus. There is no “Eastern Orthodoxy” or “Roman Catholicism” at this point. There is the Catholic Church from which today's Orthodox and Catholics claim descent. At this time, Catholic is as much still an adjective describing the universality of the Church of Christ as it is a noun, a name.
To follow Christ at this time was to belong to the Unified Church.
He was contrasting Christian faith with apostasy, with abandonment of Christ to follow the demonic religion given by Satan to his prophet, mohammed.
After hearing this, the moslems validate all the saint has said to them about their evil religion. For this, the moslems then killed him.
Well said.