Today we take solemn oaths by «placing one hand on the Bible.» (redeeminggod.com)
But they declared that all their fault or error amounted to was the custom of meeting on certain days before daybreak and singing a chant to Christ as to a god, taking turns, and binding themselves by solemn oath not to commit any crimes.2 (religion-online.org)
That was apparently how the ancient Hebrews took solemn oaths because of the blessedness of the «seed» which God had promised to multiply to Abraham and his descendants. (redeeminggod.com)