- That said, why do so many people say that He began His public ministry in
I don't think they are necessarily saying that, except in the sense that Our Lord's birthday may, or some say, probably is, off by about 4 years.
In that case, it would be correct to say the Incarnate God-Man, Jesus, started His Divine Ministry when he was born.
The posting will give you the math behind this.
I hope this helps,