Here's why and some options:
* Origin: Thaddeus is a Greek name, and it's likely that most French speakers would not recognize it.
* Alternatives: You could use the French equivalent of the name's meaning:
* Thaddée: This is the French form of the Greek name Thaddeus, which means "gift of God."
* Théodore: This is a French name meaning "gift of God" as well.
Ultimately, the best way to say Thaddeus in French would depend on the context and your preference. You could either use the name "Thaddée" or explain the meaning and use a French name with a similar meaning.