John Calvin teaches that in order to know what the image of God is, we simply need to look at how we are being renewed in the image of our Creator as believers.
By doing this we can see that the image of God in man is primarily his righteousness, holiness, and intelligence. These were all significantly marred and disfigured (though not lost) when Adam fell. These are also what are being restored as we are renewed in the image of God.
- and to put on the new self, created after the likeness of God in true righteousness and holiness. (Ephesians 4:24, ESV)
- and have put on the new self, which is being renewed in knowledge after the image of its creator. (Colossians 3:10, ESV)
Institutes Book 1, Ch. 15, Sec. 4.