Why Is Man Commanded To Obey The Gospel If he Has No Ability to do so?

Can the stony heart but choose to resist the stroke? There is not only an inability—but an enmity and obstinacy in man's will by nature. God knows, O natural man, whether you know it or not, that "you are obstinate, and your neck is an iron sinew, and your brow brass," Isaiah 48:4, and cannot be overcome—but by him, who has "broken the gates of brass, and cut the bars of iron in sunder." Hence, commonly speaking, there is such hard work in converting a sinner. Sometimes he seems to be caught in the net of the gospel; yet quickly he slips away again. Thomas Boston. Human Nature In Its Fourfold State