The Duty of Man is to Repent and Believe the Gospel
Imperatives (Commands) only demonstrate man's duty, not his ability. You might as well say that God's command to love him with all our heart is possible since God commands it... But the Bible teaches us something very different. Paul says after the fall the purpose of the commands is not to tell us our ability, but to to reveal our inability, our sin. "Through the law comes knowledge of sin. (Rom 3:19, 20)
Repent and believe the gospel is man's duty, his responsibility. He must believe in Jesus Christ to be justified. The weight of the command to believe must be impressed upon all men (Acts 17:30; 1 John 3:23). But again, the command to believe is no easier to do than the command to love God with all our heart. As Jesus himself declares, no one will believe APART FROM the grace of God in Jesus Christ, as we are united to Him by the Spirit (John 6:63, 65; 37).