quoniam

Quoniam is quom, an older spelling of cum, plus iam: literally "now that". The temporal colouring never quite leaves it, and it shapes how the word is used. Quoniam gives a reason the hearer is assumed to accept already, so it usually opens the sentence and sets up what follows. Quia supplies the reason as new information and often answers a direct why. Nam and enim explain something you have just said and cannot start a train of thought from nothing.

It takes the INDICATIVE in classical prose. If the reason is being reported as somebody else's rather than asserted as your own, the verb goes subjunctive, but that is the ordinary rule for all causal clauses and not a peculiarity of quoniam.

In late and ecclesiastical Latin it drifts. The Vulgate uses quoniam as a flat "that" after verbs of saying and knowing, where classical Latin wants an accusative and infinitive. If a text you are reading has quoniam where you expect "that", you are almost certainly reading post-classical Latin.