There are 2 theories: agia and vercike.
Agia thinks that Ken was kidnapped because vercike doesn’t want the sun and moon gods together because that would kill him like the prophecy claims (or rather the church claims).
Vercike thinks that the church doesn’t care that the sun and moon gods will die because the prophecy they claim to have decoded is actually wrong. Ken got kidnapped so that vercike can force him to remember his original life (as a god) and therefore stop the false prophecy by announcing the original one.
The main thing to note is that all 3 were present in the original life. Agia never liked vercike and vercike never liked agia. Both love ken and both are jealous of the other. Ken is the only one who actually can decide what to do since he loves both of them (agia as a lover and vercike as his child).
Hope this helps!
sooo why did versicke kidnap Ken? I didn't get the point..