Trinity
We must reference the term "trinity" in relation to Tertullian because Jesus never mentioned the term "trinity".
Yes, "He who denies the Trinity loses his soul; he who tries to explain the Trinity loses his mind." Tertullian
Tertullian is not insisting people must "believe in" the Trinity but he is saying we need to not "deny" whatever Tertullian means by the the term.
What does it mean to deny what the word "trinity" meant to Tertullian?
Which brings me to ask why do you think people can be "united" by believing in something Jesus never mentioned, the "trinity" deny the very things he directed and even commanded.
Certainly there is someone we identify as God who has a personality that includes that of a Father, Creator, law maker, and "ruling judge" which is the definition of the Hebrew and Greek terms we translated into the word "God" or "gods".
Jesus had the personality of an obedient son, loving brother, and a sacrificing husband.
The Holy Spirit would include the spirit or life of the Father and Son for the purpose of comforting us. It is like the tree of life in the garden when man walked with God.
We can all be fathers, sons, and comforters (or mothers, daughters etc.).
We can all be givers of life, self sacrificing, if we let the Holy Spirit dwell within us. If anyone deny or reject any aspect of the personality and character of the Father, Son, or Holy Spirit in our life and soul, though we say we believe in the "Trinity" he "loses his soul" as Tertullian professed.
It is a strong delusion to think "religion" is what you "think" about "God" or the "Trinity".
For centuries the word "religion" was the "performance of a duty". If you refuse or deny the "performance of that duty to God" imposed by the righteousness of the Spirit of the Father and Son you deny all.
Three in the Bible
Matthew 28:19 "Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost:"
This one of the verses that have propulgated the doctrines of the "Trinity" that seem to have divided rather than united people who say they want to "follow" Christ.
If we just read the rest of the sentence Jesus explains his doctrines on these three personalities of "the Father, and of the Son, and of the Holy Ghost".
Matthew 28:20 "Teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you alway, even unto the end of the world. Amen."
Jesus is not talking about water "Baptism". Jesus never did water baptizing and Matthew explained His form of baptism by reporting what "John the Baptist" explained about it in Matthew 3:11 "I indeed baptize you with water unto repentance: but he that cometh after me is mightier than I, whose shoes I am not worthy to bear: he shall baptize you with the Holy Ghost, and with fire:"
Again his explation is continued in the rest of the sentence, 12 "Whose fan is in his hand, and he will throughly purge his floor, and gather his wheat into the garner, but he will burn up the chaff with unquenchable fire."
This is a a metaphor speaking of the separation of the wheat from the chaff on the threshingfloor. The wind of His fan or spirit of His way comes with the "Teaching them to observe all things whatsoever I have commanded you".
The chaff is burned because it contains the weed seeds that grow up amongst the wheat which represents the true followers of Christ.
It seems counter productive to require everyone to state they believe in something that they do not understand and according to what Tertullian says, "he who tries to explain the Trinity loses his mind."
It also seems a bit hypocritical to press the issue of requiring people to say they believe in "the Trinity" when Jesus never even mentioned the termand those often do have clearly neglected in the actual instructions, if not failed the people all together by not "Teaching them to observe all things whatsoever" he has commanded us.