In part, Thomas Hardy is here describing the movement within the Church of England known as ritualism, in which great care was taken in the outward and aesthetic aspects of the church service. On another level, he is making fun of what might be an evangelical preacher who pretends to spontaneity in preaching but has actually memorized each gesture and prepared it carefully.
What makes the poem interesting is that on the one hand, the preacher appears to be effective. On the other hand, we, like the student, are surprised to see the preacher practising his craft. But that makes us ask a third question: if a student needs to study and prepare (in Bible and other classes) to do well, shouldn’t a preacher prepare every minor detail of a sermon to be as effective as possible? This is a question in which Hardy is reprising an issue raised in Augustine’s De Doctrina Christiana about the use of oratorical technique in preaching.