Merrick struggles throughout the play to find a normal life, given the enormous growths on his head that lead to him being part of a freak show for much of his early life. He is taken advantage of frequently and struggles to have real friendships and a sense of purpose.
But he has very deep religious beliefs, and part of the way he expresses them is in the way he studies and eventually constructs a model of St. Phillip's church. The model building coincides with his progression and the fact that he has finally found some human companionship and has found people who are willing to listen to him and consider him human.
As he continues the construction of the church, his understanding of and relationships with the people around him deepen and he appears to be moving towards a somewhat complete life including a sexual awakening through his relationship and conversations with Mrs. Kendal.
He finishes the model just before he dies, and in some ways the model might then be interpreted as a model of his life. As it reaches a completed state, his purpose and the conflict are resolved and he is released from the difficulties of his mortal body and its disfigurements.