Early Christians believed Jesus was the son of God, a god in his own right (sometimes rationalized as part of the Holy Trinity or part of the godhead.) But a God, right? Right?
Actually, you're wrong.
Although the gospels portray Christ as "the Son of God" it is not the same as saying he was actually God.
Being mindful of the fact that the gospels were not written in the 21st century with our current understandings of Christianity, one needs to place the gospels in the context of the time they were written -
a 1st century context.
The Old Testament speaks of many groups and persons who were considered to be sons of god, but that does not mean they were recognized as gods:
The King of Israel was called "the Son of God." The reference was that of a parent to a child.
The nation of Israel was recognized as "the Son of God." That didn't mean the nation was divine. It meant that Israel was the people through which God worked his will on earth.
In the gospels of Matthew, Mark, and Luke, Jesus is recognized in this sense - a man that God had a close relationship with, through whom God worked his purposes.
In pre-Pauline traditions Jesus is said to have BECOME the Son of God, NOT God himself.
That in a nutshell is the earliest understanding of Jesus and his relationship with God. The reference in question was not all that uncommon.
I understand that atheists do not choose to develop a deeper understanding of matters like this. That too is not all that uncommon.