In this excerpt from his classic Lectures in Systematic Theology, Henry Thiessen explains the concept of prevenient grace:1
All Christians are agreed that God has decreed to save men, but not all are agreed as to how He does this. We must, in this connection, particularly remember that God must take the initiative in salvation, that man, even in his present helpless state, is really responsible, and that God’s decrees are not based on caprice or arbitrary will, but on His wise and holy counsel. To our mind, the following things seem to be involved in the decree to save sinners: