Why, after reading over 600 volumes in 4 years would the lawyer spend one year reading the New Testament?

We can infer from the information provided in the text, the prisoner was seeking salvation.... and afterlife. His study of theology was based upon hope.

It seemed strange to the banker that a man who in four years had mastered six hundred learned volumes should waste nearly a year over one thin book easy of comprehension. Theology and histories of religion followed the Gospels.

In the last two years of his confinement the prisoner read an immense quantity of books quite indiscriminately. At one time he was busy with the natural sciences, then he would ask for Byron or Shakespeare. There were notes in which he demanded at the same time books on chemistry, and a manual of medicine, and a novel, and some treatise on philosophy or theology. His reading suggested a man swimming in the sea among the wreckage of his ship, and trying to save his life by greedily clutching first at one spar and then at another.