Generally people say "Catholic", but really mean "Roman Catholic". Roman Catholics believe that they were the Christians before denominations as seen here:
THE ORIGINATOR OF PROTESTANT “CHRISTIANITY”
If you are Catholic, you know that your religion was founded in the
year 33 by Jesus Christ, the Son of God, true God and true man; and
that this one Church, to which people must belong to be saved, will
exist until the end of time.
What I would say next is better articulated here:
The exact date of the beginning of the Roman Catholic Church
is indeterminable. While the belief system recognized as Christianity
is in place by the first century, institutional structures developed
over time. Nor is it possible to distinguish Catholicism as a separate
tradition until it can be differentiated from other Christian
traditions (most notably, Eastern Orthodoxy and Protestantism).
The name "Roman Catholic" is generally not used unto the Council of Trent (1545-1563), it clearly didn't exist before Council of Nicea (325). It was use to mostly distinguish "the others" than to identify themselves.
Trying to pin point a date leads us into murky waters. For example, when was the actual split between Judaism and Christianity? And who considered who different first? Was it when Christians consider themselves distinct from Judaism or when Judaism considered Christians distinct OR when they both agreed (did they ever)? A similar set of questions appears when you ask when Protestantism split from Catholicism. Did Catholics consider Protestants different or did Protestants consider themselves different or do they agree?
Back at the ranch, there is no way to pin point the exact day that Catholicism appeared as it was transition, not a quantum leap.