Practical Atheism is a fancy way of pointing out a terrible sin.  It is the sin of casting aside the collective moral imperative of the Gospel.  The Gospel is God's message of salvation to this age.  Those who receive Jesus as their Savior have been delivered from eternal death. After salvation believers are to grow in the knowledge of our Lord and Savior.  However, when believers stop growing in the Faith, and they allow sin to take over they may start to pretend that God is not there.  Practical atheism is a highly destructive sin.