Full Idea
Once people are declared to be images of God, just by virtue of minimal humanity, they have, therefore, no greater individual excellence to aspire to, and their purpose became one of obligation, that is, obedience to God's will.
Gist of Idea
If we are made in God's image, pursuit of excellence is replaced by duty to obey God
Source
Richard Taylor (Virtue Ethics: an Introduction [2002], Ch.2)
Book Reference
Taylor,Richard: 'Virtue Ethics: an Introduction' [Prometheus 2002], p.8
A Reaction
An interesting and plausible historical analysis. There is a second motivation for the change, though, in Grotius's desire to develop a more legalistic morality, focusing on actions rather than character. Taylor's point is more interesting, though.