Some claim it would be unjust of God to command us to do something we are unable to do.That is an odd claim. Isn't that why we need grace to begin with? ... because we are morally bankrupt?, impotent to carry out the works of the Law? The command "Love the Lord your God with all your heart..." Does anyone naturally love God with all their heart? No. We are impotent to obey this command in the flesh. Does that make God unjust for asking us to conform to his holy standards? No, of course not. The commands of God are righteous and holy and we only have ourselves to blame for rejecting them ... not to mention that the purpose of the law is to reveal sin (Rom 3:19-20)
But lets bring this same question to the summons to believe the gospel. We all agree that the choice of right or wrong is put before every person. "Believe in the Lord Jesus Christ and you will be saved." Yet, even if we did not inherit original sin from Adam, the moment we choose to sin in our personal lives against an infinitely holy God, we would render ourselves dead in sin, unspiritual creatures of the flesh, owing a sin-debt we cannot repay. Any good will or moral ability to see the goodness, beauty and excellency of Christ has been destroyed by sin. As a result, "...there is no one who seeks God" (Rom 3:11) Now, in every day life, if you owe a debt you cannot repay... are you still responsible to repay the debt?