We generally consider any early church father Catholic, as well as, of course, Orthodox. The precedent was set for St. Ignatius in the Letter to the Smyrnaeans.
In the case of Emperor Constantine it is problematic because he was not baptized till his deathbed. Is it why you consider it improper to call him Catholic?
Thanks for your elaboration.
I don't consider it improper. I consider it misleading visa vis non-catholic/orthodox. To us, as to you (plural), the Church is catholic and the faith orthodox, so members of the Church are Catholic and Orthodox at the same time.
All the popes before the Schism were orthodox as far as we are concerned (i.e. professing the Orthodox Faith, as per Creed). Any Roman Catholic or eastern Orthodox will automatically consider all Fathers of the Church both 'catholic' and 'orthodox' belonging to the one and the same undivided Church.
But to the Protestant ears when they hear "Constatine was Catholic" they say "aha, he made the Catholic Church..." In the 19th century, the Orthodox Church was much more adamant about stressing its catholicism, which is apart of the official name of the Church (Orthodox Catholic Church), but the term "catholic" has indelibly been etched into everyone's mind as being Latin and Roman. Perhaps Latins and Romans need to be more aware of how their message comes across.