In other words, this life is the only opportunity we will ever have to serve God and do good. Therefore, as Matthew Henry says, when that opportunity to serve is open we are to "... do it with our might, with care, vigour, and resolution, whatever difficulties and discouragements we may meet with in it. Harvest-days are busy days; and we must make hay while the sun shines. Serving God and working out our salvation must be done with all that is within us, and all little enough"
Can a poor sinner serve the Lord with fervency and passion? One might ask the question differently: How can a sinner saved by God's sovereign grace choose to NOT serve his Saviour with a fervent spirit?
The famous Scottish Covenanter , Donald Cargill testified as he came to his death, "I have been a man of great sins, but he has been a God of great mercies. And now, through his mercies I have a conscience as sound and quiet, as if I had never sinned."
Past failures must not keep any one from serving the Lord. Take them to the cross and there confess and repent of the sin, and then rise with fresh determination to go on serving the Lord fervently. No one has more cause to serve the Lord fervently, than those who are most conscious of their weakness and have been forgiven "all their iniquities" (see Psalm 103:1-4).