In a nutshell, a thesis statement is a single sentence that will express the main idea of your entire essay. It makes a claim that directly answers a question.
To come up with a great thesis statement, the first thing you need to do is figure out what the question that requires answering is. You haven’t said that you’ve been given a specific topic, so I’m going to assume that you get to pick your own topic.
The Gospel according to St. Mark is the oldest account of the life of Jesus Christ, and is therefore considered to be the most reliable. It is an account of Jesus’ adult life, and does not cover the story of his birth.
One of the topics to which Mark pays much attention is Jesus’ miracles. If your essay was to be whether there was a miraculous or supernatural element to Jesus, then a suitable thesis statement could be “A study of the various miracles performed by Jesus showcases clear evidence that he either has supernatural powers or is, as he claimed, the son of God.”
Another essay idea would be to focus on the topic of betrayal, as the story of Jesus’ crucifixion begins with him being betrayed by Judas Iscariot, who was one of his disciples and closest friends. Here an example of a great thesis statement would be “The death of Jesus Christ was proof that betrayal comes not always from our enemies, but sometimes from those closest to us.”