About Kristy Lashbaugh Expertise I can answer questions about the meaning, usage, and source of English vocabulary. I have no formal training in Latin. My experience with Latin consists of Latin Morphemes in word study. For Latin translations (or other translations) I recommend finding another expert. I'll do what I can to help, but am limited in these areas.
Experience English degree with extensive background in romance languages including French and Spanish
Education/Credentials B.A. English, Additional Coursework in psychology, philosophy.
Question I understood that to say "I swear" around 1850 would actually be considered swearing. Is that correct and why?
Am assuming it was because it was swearing to God and that was not considered something to take lightly.
Answer I went through the listings in the Oxford English Dictionary, which is not exhaustive, and found that "I swear" and "to swear" in the context which you've asked about was quite common as an oath in the mid-nineteenth century. It appears in Dickens' writing. I found a quote referring to a character "swearing at" or cursing someone in Disraeli's writing.
I would say that based on the evidence, "I swear" was not considered cursring in and of itself, but what one was swearing or what the swearing was in relation to could make it unacceptable.