A large part of our self-deception is a misjudgment of what's really important. Even Christians can become practical atheists, leaving God out of our daily work and plans.
Gregory described it like this. . .
For the craftsman considers that the Divine assistance is quite useless for the work he has in mind. Therefore he leaves prayer aside and places all his hopes in his hands, without remembering Him who has given him his hands. In the same way someone who carefully composes a speech does not think of Him who has given him speech. . . . everyone devotes all his energy to the work he has in hand, forgetting completely the work of prayer because he thinks that the time he gives to God is lost to the work he has purposed to do.
Thus, it comes about that life is so full of sin. . . . Everyone keeps forgetting God, and people do not count prayer among the good things worth pursuing. Covetousness enters together with trade; but covetousness is idolatry.
Gregory diagnoses this busy forgetfulness as a "spiritual sickness" because it leaves Christians vulnerable to the schemes of the evil one. If this was a danger in his day how much more so in a society characterized by the urgent imperatives of the moment. Rightly ordered reality -- which is what worship is (thank you for reminding me of that Pastor Dan!) -- reorders our priorities. Instead of forgetting God we begin to practice a constant awareness of him.
For when the consciousness of God is firmly established in the heart, the devices of the devil remain sterile.
Quotes by Gregory of Nyssa from pp. 146-7 of Christopher A. Hall, Worshiping With the Church Fathers (InterVarsity, 2009)