If the Church is supposed to be the pillar of truth why do they teach the trinity not mentioned in the bible?

Trinity is a word we made up to explain Father Son and Holy Spirit. Look at the word and its make up tri meaning three and unity meaning working as one. Three entities working as one. Just because the word is not in the Bible doesn't mean it isn't useful. The word Christian isn't in the Bible either and yet you use it to describe a group of people, Don't you?