expression, godan verb (-u)
pledge before God; swear before God
A formal, often religious or solemn expression used when making a vow or oath in the presence of God.
彼は神前に誓って真実を語ると言った。
He said he would tell the truth, swearing before God.
誓う is the general verb for 'to swear' or 'to vow'; 神前に誓う specifically adds the location 'before God' and carries a more solemn, religious tone.
Compound of 神前 (しんぜん, 'before God') and 誓う (ちかう, 'to swear'). The exact historical derivation is uncertain, but the expression is conventionally associated with solemn oaths in religious contexts.