Ok kiddo, so there was this big meeting called the First Council of Constantinople a long, long time ago. It was like when all the grown-ups got together to talk about important things.
Now, some people believed in different things about God, and they were always arguing about it. Some thought there was only one God, and others thought there were three Gods all in one.
At the meeting, the big group of grown-ups decided that there was only one God and that Jesus was important too, but he wasn’t really a God in the same way as God the Father. They also talked about something called the Holy Spirit, which is kinda like a special power that comes from God.
They wrote all of this down in something called a Creed, which is like a special set of rules about what people should believe. This Creed is still used today in a lot of churches.
So basically, the First Council of Constantinople was a meeting where the grown-ups decided and wrote down what they thought was the right way to believe about God and Jesus. It helped people come to an agreement and understand each other better.