Question

Why is Winthrop’s sermon an effective way to unite the members of his audience?

Answer 1

I think the answer is none of the above.

The closest answer would be that his audience was used to sermons. But it was not because they had a religious lifestyle. One can have a religious lifestyle without frequently listening to sermons. 

I would say that his sermon was an effective way of uniting the members of his audience because he preached about their serving God as a heterogeneous community, that is a community composed of different people - some were rich, some were poor, but all of them together were to serve God and function as a community based on God's principles. When the Puritans settled in America, they didn't have access to the written word, so sermons were the best means of interpreting God's will and directing their mission to establish what they deemed a perfect society. And they could do it only as a community in which every member had a function.