No; it isn't.
It's the Bible's
He merely dug out the Scriptural proof and put it all together. Then, after the Church saw the Truth of it, adopted a concise group of sentences that boiled down the essential beliefs into an easily understandable Creed.
Credo - Belief.
Kinda like the LDS Organization's set: The Articles of Faith.
I believe the Bishop of Rome was a better authority on the doctrine that Tertulian was at that time.
The Bishop if Rome rejected it as heresy. Tertulian left the Church where he wrote it and joined an off shoot.
How is it that 150 years later his heretical writing suddenly became correct?
It was adopted by Constantine because those he had convened could not agree and Tertulian's writings were such that you could read what you wanted in it.
It was adopted because it was abstract and didn't make explanations with any clarity.
For you to say they adopted a group of sentences is revisionist.
However, you certainly have the right to believe what you want. If you want to be a follower of a Pagan Lawyer you may.
I have had discussions with Lawyers concerning rulings by Judges. Their standard way of answering when confrunted with the facts is to say "Well, that is not the way I read it." and then go on and say "The way I read it...." and make it up just as the Lawyer Tertulian did.
Just because the Bishop of Rome rejected it as heresy and it was not accepted for 150 years doesn't affect your right to follow his teachings if you want.