Common grace is an operation of the Holy Spirit, based on the atonement of Christ and God’s merciful and benevolent attitude toward all, by which He immediately or through secondary causation restrains the effects of sin and enables the positive accomplishment and performance of civic righteousness and good among all people.
McCune, A Systematic Theology of Biblical Christianity, 2:297.
Grace is basically God’s condescending, unmerited favor. Common grace is favor shown to all men in common. God was under no constraint or compulsion of necessity to show this favor. He could justly have left the world to the full, unrestrained and unmitigated effects of sin. That God arrested the progress of these just desirets is all of grace.
McCune, A Systematic Theology of Biblical Christianity, 2:300.