What is the best way to start an essay on the Bible?

To start any essay, you need an attention getting hook.  It's the first sentence(s) that you put down on paper, and it serves to grab reader attention.  This must be done because bored readers aren't motivated readers.  Unmotivated readers stop reading or don't actively engage with the rest of the paper.  I recommend 1 of 4 possible attention-getters.  
Use a quote.  This would work well with an essay on your chosen text because it is filled with quotable passages.  Quotes tend to engage the reader because the reader wants to know why that particular quote is more important than anything you might have written in your own words.  
Ask a question.  This is engaging because readers can't help but think of possible answers.  Readers also wonder what your answer might be.  
Make a bold or even controversial statement.  This will grab the reader's attention because it aims to get an emotional response out of a reader.  Anybody that is emotionally engaged with a piece tends to stick with it for longer.  
Use a definition.  Definitions have the risk of falling flat and being boring, so choose the definition carefully.  I have no idea what your essay is going to be about, but if it is a thematic analysis, you could define the meaning of that theme.  This definition will engage logic centers of the reader's brain while serving to steer them toward the thesis and point of the paper. 
Once the attention-getter is nailed down, the next thing to do is write a thesis.  The entire biblical scripture is a big book, and it has lots of characters and themes, so there is no shortage of potential topics.  The key to a thesis is that it will make an argument.  Don't make a statement of fact.  The statement needs to be something that you intend to prove.  I usually recommend a theme or character analysis for a literary thesis.  For example, "Although seemingly at odds with each other, the Old Testament scriptures do indeed mirror the words of Christ."