Scriptural vs. non-scriptural

[DeeDee]

How do non-scriptural things become church dogma? Like the assupmtion of Mary for instance?

[Laudate_Dominum]

All dogma is scriptural. However, the authoritative teachings of the Church need not be explicitly taught in Scripture, but they are rooted in Scripture. And Scripture understood in light of Sacred Tradition.
For example, if you look at doctrines authoritatively proclaimed in the early Church councils, for example regarding the nature of the Trinity, or Christ's divinity. These are things that are contained in Scripture, but do not always include an exhaustive description.