What is a good way to grab a reader's attention if I'm writing an essay about the religion in the Victorian era, or just religion in general?

First, it is important to decide what your specific focus will be. If you do choose to write about religion in the Victorian era, this focus will be important when deciding on an attention-grabbing introduction. In Victorian England and the United States, Protestant Christianity was the most common religion. In Victorian England, most Christians belonged to the Church of England. During this time in the United States, many people were Episcopalian. The largest religion outside of Christianity was Judaism. With these facts in mind, you can consider your options to grab attention.

The purpose of a grabber at the beginning of an essay is to draw in your reader. You want your reader to be interested in your topic. You want them to read more of your essay.  

An interesting question could be a good way to grab a reader's attention. You want it to be a question that causes your reader to think. Here's an example: "Did you know most religious people during the Victorian era were Protestant Christians?"

You can start with a fascinating quote about religion during Victorian times.  

Beginning with a short story or fact can draw in your reader. For example: "Almost every religious person during the Victorian era was a Protestant Christian." A short story related to religion in Victorian times can also make your reader interested in the topic.

Data regarding religion in Victorian times will show the vast majority of religious people were Protestants. You can state what percentage of people were religious, and then identify how many of those were Protestants. You will probably find data sorted by country.