I think the apostles are considered the first bishops, though they were also priests!
In any case, Christ conferred the authority on the bishops -- as the Church. He didn't go into the practical details of how that authority should be exercised. One imagines that in a community of a few hundred, the bishop could hear all the confessions. Even in the West, priests have the authority only as derived from the bishop (with some exceptions).
It seems to me we heard in high school that in the very early Church (first few years or decades?), the practice was for public confession (I assume "public" in the sense of the Christian community, not the world at large), with public penance frequently required. I don't know what "formula" might have been used or who spoke what words and, if it's known, I'm unaware of it.
“It seems to me we heard in high school that in the very early Church (first few years or decades?), the practice was for public confession (I assume “public” in the sense of the Christian community, not the world at large), with public penance frequently required.”
Public confession was the practice for hundreds of years, probably for the first millennium, though for “secret sins” there were rare “secret confessions”.
I think the link to O Gladsome Light (Phos Ilaron) is more interesting. Imagine a hymn in use today which expresses the earliest Christian form of praise and understanding of an attribute of God...that +Basil the Great called the most ancient of hymns and that we still use to this very day!