Question

What is the main message of sinners in the hands of an angry god

Answer 1

to warn his congregation in particular, and presumably, by extension, his nation as a whole, that they must repent of their sinful ways and turn to God for forgiveness before it is too late

Answer 2

His purpose in delivering the sermon is to warn his congregation in particular, and presumably, by extension, his nation as a whole, that they must repent of their sinful ways and turn to God for forgiveness before it is too late.