1
general::
Be converted; also, decide to behave in an upright, ethical way. For example, After the children were born, John got religion and joined the church, or After years of total selfishness, she suddenly got religion and is doing all kinds of volunteer work. [Second half of 1700s]
American Heritage Idioms
2
general::
Phrase(s): get religion
Fig. to become serious (about something), usually after a powerful experience. (Sometimes literal.) • When I had an automobile accident, I really got religion. Now I’m a very safe driver.
McGrawhill's American Idioms And Phrasal Verbs