expression
I swear to God; as God is my witness; cross my heart
Used to emphasize the truthfulness or seriousness of a statement, similar to English oaths. Can be used in both religious and secular contexts, though it carries a strong, somewhat dramatic tone.
神に誓って、私は嘘をついていません。
I swear to God, I'm not lying.
神に誓って、もう二度とあんなことはしません。
As God is my witness, I'll never do that again.