According to John of the Cross in Dark Night of the Soul the third sin is Spiritual Luxury. He even claims that all other sins come from this one.
"Here’s what happens: a soul which is deep in prayer may experience profound temptation and find itself powerless to prevent them."
John cites three causes for this, the weakness of the flesh, the devil, and our own fear of impurity.
This one I am still processing. I am struggling to comprehend this connection between a feeling of spiritual fullness and the need for prayer.
As best as I understand, when the soul experiences spiritual luxury you cease wanting to pray for three main reasons.
1. The flesh stirs up too much physical response and you pray for the wrong reasons.
2. The devil insights disturbances within the prayer time and for fear, you stop praying.
3. The person’s own fear of impure thoughts will keep them from wanting to pray and lead to distraction.
When you enter the "dark night, the flesh will be quieted, the devil will be silent and all fear will subside." The point is, prayer is th most powerful connection between a soul and God and the enemy and even our own flesh wants to block that connection.
Romans 8:28 And we know that in all things God works for the good of those who love him, who have been called according to his purpose. 29 For those God foreknew he also predestined to be conformed to the image of his Son, that he might be the firstborn among many brothers and sisters. 30 And those he predestined, he also called; those he called, he also justified; those he justified, he also glorified. – Romans 8:28-30