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

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.

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