Where are we going with this? The purification of the heart so that the Spirit of God may dwell in man. Fr. John Romanides explains it:

So what is purification? Purification means that all thoughts leave the nous and only one remains. This thought is the remembrance of God, the impression on man’s heart of the remembrance of God, which now works twenty-four hours a day. We clean the nous thoroughly. We throw out of the nous all concepts and thoughts. We make it a nous with a single thought. It has one thought instead of many: single-thought prayer begins. This is the cure of the nous and the return of the memory – the constant remembrance of God. The nous recovers the memory it had lost. Thus purification for the Fathers is an ascetic state which man reaches after a struggle, not simply to avoid sin – because avoiding sin is an integral part of asceticism – but to purify the heart.

Purification of the heart does not only mean avoiding sin. Purification of the heart means that thoughts leave the heart and go to the rational faculty, leaving the heart free of thoughts. It is left with one thought alone – prayer. This is extremely important. Once it is acquired, this is called prayer of the heart or single-thought prayer. This one thought, the prayer itself, constitutes the constant remembrance of God. This is purification of the heart according to the Fathers. It is not simply avoidance of sin.

When the Fathers of the Church speak about purification of the nous, they mean that the nous should be emptied of good thoughts as well as bad ones. When the nous is empty and the passions have been defeated, the Holy Spirit comes and visits. Then the Spirit takes it upon Himself to pray in man’s heart.

The Holy Spirit’s prayer cannot come to someone unless his nous has previously been emptied. This is the start of man’s therapeutic treatment. The single-thought prayer means the existence of only one thought, which is the remembrance of God: “Lord Jesus Christ have mercy on me a sinner.” [Source: Mystagogy]

The mind must descend into the heart the Fathers of the Church teach

Where are we going with this (6th paragraph from the bottom)? What is the objective?

Lord, have mercy.