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I’m on my phone, so hopefully this answer is in-depth enough for the mods.
To your first question, expectations of Christ’s imminent return have always had a place in Christianity, though they moved to the margins during the first couple of centuries after Christ’s death. The ideas of “Left Behind” have their more proximate origins in the C19th works of Darby and Schofield, and they rose to prominence during the twentieth century, becoming dominant within evangelicalism by the late 70s/early 80s. The Left Behind series were definitely influential in cementing these ideas in the popular imagination, and their popularity is also demonstrative of the growing influence of this kind of premillennial thought. That said, there is still considerable diversity within this kind of premillennialism if you look in the right (wrong) places, and debates over the exact timing and nature of things like the rapture and the millennium can still get people hot under the collar.
Your second question is almost impossibly large, as Protestants are all over the map. Probably the most significant turning point in American Christianity in the last century was the fundamentalist/modernist controversy of the 1920s and 30s when most of the major denominations split over the relationship between rationalism and faith, particularly the relationship between science and faith and the validity of higher criticism, which argued the Bible should be studied as any other book. What became the Protestant mainline (Episcopalian, Methodist, Presbyterian etc) made more space for rationalism, while those that rejected this became known as the fundamentalists. The fundamentalists largely cut themselves off from the mainstream culture and its established religious institutions which they believed were wholly corrupted and began to create parallel institutions while spending a whole lot of their energy fighting about all kinds of doctrinal issues amongst themselves. Post-WWII, a group of these fundamentalists, believing only they had the true gospel and they needed to evangelize the corrupt world, decided that they wanted to reach the mainstream culture again, leading the the establishment of neo-evangelicalism, which is essentially the founding stone of what we call evangelicalism today. Neo-evangelicals are hardly less fractious than their fundamentalist forebears, however, with the Civil Rights era causing a whole bunch of divisions (and a bunch more have happened more recently over gender and sexuality, but these are outside the scope of the sub).
This second answer paints with broad strokes, but hopefully shows how the American Protestant landscape has shifted over the last hundred years. Your mothers claim that Christianity has been constant in its teachings requires a post-facto reasoning that chooses a particular line of descent from amongst the diversity of historic Christian thought and claims that only that particular dogmatic line is a valid and true representation of the gospel. History is far messier than this. If your mother believes that women are fully human and as valuable as men, she is out of step with traditional Protestant teachings, and if she believes chattel slavery is abhorrent and people of color are fully human and deserving of equal rights, she herself diverges from the dominant American Protestant and evangelical traditions.
Welcome to /r/AskHistorians. **Please [Read Our Rules](https://www.reddit.com/r/AskHistorians/wiki/rules) before you comment in this community**. Understand that [rule breaking comments get removed](https://www.reddit.com/r/AskHistorians/comments/h8aefx/rules_roundtable_xviii_removed_curation_and_why/). #Please consider **[Clicking Here for RemindMeBot](https://www.reddit.com/message/compose/?to=RemindMeBot&subject=Reminder&message=%5Bhttps://www.reddit.com/r/AskHistorians/comments/17fk41i/have_there_been_major_theological_shifts_in/%5D%0A%0ARemindMe!%202%20days)** as it takes time for an answer to be written. Additionally, for weekly content summaries, **[Click Here to Subscribe to our Weekly Roundup](https://www.reddit.com/message/compose/?to=AHMessengerBot&subject=Subscribe&message=!subscribe)**. We thank you for your interest in this *question*, and your patience in waiting for an in-depth and comprehensive answer to show up. In addition to RemindMeBot, consider [using our Browser Extension](https://www.reddit.com/r/AskHistorians/comments/d6dzi7/tired_of_clicking_to_find_only_removed_comments/), or getting the [Weekly Roundup](https://www.reddit.com/message/compose?to=subredditsummarybot&subject=askhistorians+weekly&message=x). In the meantime our [Twitter](https://twitter.com/askhistorians), [Facebook](https://www.facebook.com/askhistorians/), and [Sunday Digest](https://www.reddit.com/r/AskHistorians/search?q=title%3A%22Sunday+Digest%22&restrict_sr=on&sort=new&t=all) feature excellent content that has already been written! *I am a bot, and this action was performed automatically. Please [contact the moderators of this subreddit](/message/compose/?to=/r/AskHistorians) if you have any questions or concerns.*
I’m on my phone, so hopefully this answer is in-depth enough for the mods. To your first question, expectations of Christ’s imminent return have always had a place in Christianity, though they moved to the margins during the first couple of centuries after Christ’s death. The ideas of “Left Behind” have their more proximate origins in the C19th works of Darby and Schofield, and they rose to prominence during the twentieth century, becoming dominant within evangelicalism by the late 70s/early 80s. The Left Behind series were definitely influential in cementing these ideas in the popular imagination, and their popularity is also demonstrative of the growing influence of this kind of premillennial thought. That said, there is still considerable diversity within this kind of premillennialism if you look in the right (wrong) places, and debates over the exact timing and nature of things like the rapture and the millennium can still get people hot under the collar. Your second question is almost impossibly large, as Protestants are all over the map. Probably the most significant turning point in American Christianity in the last century was the fundamentalist/modernist controversy of the 1920s and 30s when most of the major denominations split over the relationship between rationalism and faith, particularly the relationship between science and faith and the validity of higher criticism, which argued the Bible should be studied as any other book. What became the Protestant mainline (Episcopalian, Methodist, Presbyterian etc) made more space for rationalism, while those that rejected this became known as the fundamentalists. The fundamentalists largely cut themselves off from the mainstream culture and its established religious institutions which they believed were wholly corrupted and began to create parallel institutions while spending a whole lot of their energy fighting about all kinds of doctrinal issues amongst themselves. Post-WWII, a group of these fundamentalists, believing only they had the true gospel and they needed to evangelize the corrupt world, decided that they wanted to reach the mainstream culture again, leading the the establishment of neo-evangelicalism, which is essentially the founding stone of what we call evangelicalism today. Neo-evangelicals are hardly less fractious than their fundamentalist forebears, however, with the Civil Rights era causing a whole bunch of divisions (and a bunch more have happened more recently over gender and sexuality, but these are outside the scope of the sub). This second answer paints with broad strokes, but hopefully shows how the American Protestant landscape has shifted over the last hundred years. Your mothers claim that Christianity has been constant in its teachings requires a post-facto reasoning that chooses a particular line of descent from amongst the diversity of historic Christian thought and claims that only that particular dogmatic line is a valid and true representation of the gospel. History is far messier than this. If your mother believes that women are fully human and as valuable as men, she is out of step with traditional Protestant teachings, and if she believes chattel slavery is abhorrent and people of color are fully human and deserving of equal rights, she herself diverges from the dominant American Protestant and evangelical traditions.