Simply put, the Trinity is very difficult to understand, especially when understood correctly!
This has been true from the very beginning. Tertullian, often called "the father of the Trinity" for being the first person to use the Latin Trinitas, wrote in A.D. 200:
Tertullian's concern was a doctrine known as "modalism." This is the view that there is only one person in the Godhead. That one, individual God fills the role of the Father, Son, and Holy Spirit in much the same way that one actor can play three roles in the same movie. (Today it would be called oneness or "Jesus only," and it's the view of the United Pentecostal Church and Apostolic Church.)
"The simple," as Tertullian calls them, had great difficulty with the Trinity. They reasoned that if there are there persons, then there are three Gods, and they knew that Christians are monotheistic: they believe in one God.
It was just easier to be a modalist. The only problem is that modalism isn't true. There is not one divine person; there's three.
The Christian writers of the second and third century claimed that their doctrine was taught by the apostles. Irenaeus, for example, writing in A.D. 185, said:
I will use Tertullian's response to modalism as a basis for what follows. Tertullian is well-respected. He also does a superb job of thoroughly explaining the early Christian view.
Then I will show you that this view is not only consistent in all the early Christian writings, but it is also what the early Church agreed upon at the Council of Nicea. The Nicene Creed, put together by that council, is a standard of orthodoxy to this day.
Hopefully, I will do a good job, and this will be easy to understand. That's a difficult chore!
We'll take this one subject at a time: