Dogma, in the popular use of the term, means to take something as true without justification.
But what it really means, is to take one side of a dilemma without having the relevant details that would be needed for a more reasoned position. It’s not that there’s no justification at all, it’s just that full justification isn’t possible, so we’re going to make a decision and move on to different questions.
So in the early days of Christianity, one of the biggest dividers of the Church was the question over the true nature of Jesus. Was he actually God, or just a prophet? As is customary in theology, whenever one side wins, the other side is given a name so that we can divorce it from the fabric of understanding.
The side that felt that Jesus couldn’t be literally divine is called arianism, after it’s chief proponent, Arius, who was the leader of a group of early Christians. Within this position were several variants, each of which used slightly different language to describe the relationship between Jesus and God. It was a highly semantic debate that hinged on the meanings of Latin words that are extremely different than the ones we use today.
Dogma is what happens when the argument gets so loud that someone with a big voice has to come in and quiet it down. Which is what happened at the Council of Nicaea in 325, which is where the specific words that eventually became the Trinity dogma were set down.
Dogma is what happens when debate becomes political enough to force a decision. If they could have all gotten along, then there would have been no need for an edict.