Here, he is interested in speaking of God not in relation to creation, but in himself. As such, we must deny all these things that we formerly said of him. God is not really good; we only call him that because he causes good things.
The idea is this: with cataphatic theology, we have made statements about God. But now we have to rescue God talk from anthropomorphism. Making only positive predicates makes it seem that God is a metaphysical entity like all others. So, both cataphatic and apophatic are needed. They are irreconcilable opposites, and yet are reconciled on a higher plane, in a higher way of talking about God. It is called the "way of eminence". We say that God is "super good", for instance. This does not just remove the positive predicate, it goes beyond it.
This is not meant to be a fully intellectual process. Dionysius follows Nyssa here. We can only "see" how it works by means of the will, not the intellect.
This work is one that traces Moses' steps in his ascent of Mt. Sinai. As he ascends, he goes beyond all that can be known with the senses, and enters a divine darkness. The soul learns to abandon modes of prayer that rely on image, and learns an openness to God in a kind of abandonment of one's own capabilities. This will later be Eckhart'sGelassenheit and Abgescheidenheit and Julian's "abandonment".
There is a process that is mirrored in liturgy here, for Dionysius. Just as the litergy goes from songs and readings to a contemplation in silence (Eucharist), so too does Moses' ascent and the mystical path. The goal of this is ecstacy not an extraordinary experience, but having one's life centred on the beloved so that the life of the beloved is one's own. It is "suffering", in the sense that it is receptivity to the extent that the one who loves God is the vehicle of God's power and love. It is ecstatic in the sense that the self of the lover is driven out by the love of God. The negative way surrenders the soul to the unknowable God. Strictly speaking, though, this "going out" goes both ways. God goes out of himself in love to establish the creature's being, and the creature goes out of itself in love in union with its creator.
This means that the negative way is more than something that we do. It is something that we "suffer" it happens to us. The mystical state is one of passive surrender to God, brought about by an immediate contact with God.