There has recently been some discussion on the internet about the idea that being in communion with the Church or Bishop of Rome has an inherent connection with 'being a Catholic' or 'belonging to the Church'. I rather feel that some people who bullishly demand patristic references to such a concept may in fact be polemically asserting that such an idea did not exist in the early centuries. I am not interested in such. But for any who ask the question genuinely seeking an answer, I commend the following article, which argues convincingly that the Communicantes in the Roman Canon originally followed on from the reference to the Roman Pontiff in the Te igitur. It is written in German but includes a good simple summary in Latin for those who, like me, are germanically challenged.
"Te igitur" und "Communicantes" im Messkanon, Sacris Erudiri VIII, 1 (1956).
I found pp 26-35; 53-54; 65-67 particularly helpful.