Well, i don't think we had anyone take an oath for Contract until now in the Manga (including the plot covered in ongoing Part 4), nor had they been said about it specifically anywhere. And truthfully, the full scope of the Gods in this Story is only understood from later half of Part 4, so bringing it here isn't really useful as majority Manga readers wouldn't know (if it hadn't been mentioned in the Manga yet).Not sure if it's ever explicitly mention early on, but one moment I do know get that title mentioned
is with the new Zent swearing her oath:
“O Zent, blessed by the gods, declare your loyalty to the Goddess of Light, ruler of contracts. Beleuchkrone.”