The essence of idolatry is the entertainment of thoughts about God that are unworthy of Him. It begins in the mind and may be present where no overt act of worship has taken place.
He's saying that you can make an idol by thinking of God wrongly or one sidedly.
So taking God's kindness as His primary attribute and negelecting His justice is idolatry.
JD Greear says that the reason the Israelites were commanded not to make graven images, even of God, was so they wouldn't emphasise one of His attributes over the others.
