Hm...it is customary—just interesting, but then why would he have a Bible in proximity?
Question: do you think the use of the Lincoln Bible use was 2 fold—to try to make a big deal about his historic oath, and to deflect from fact perhaps that his family doesn’t own a cherished famiy Bible?
Of course there’s no requirement but God being left out is very interesting. As if God is saying “Not in my name.” Or He’s being told, “Not in your name.”