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.
I swear to God, I'm not lying.
As God is my witness, I'll never do that again.
A set phrase composed of 神 (god) + に (particle) + 誓って (te-form of 誓う, 'to swear'). The exact origin is uncertain, but it mirrors similar oath expressions found in many languages.