Union with God is realized in the vision of the Divine Being "face to face" (1 Cor 13:12), a vision called "beatific" because it carries with it the ultimate attainment of man's aspiration to truth. John Paul II, Crossing the Threshold of Hope, P.71.
When God comes, where is sleep? Where is the body? Nothing matters but His intoxicating presence. (...) To be ever conscious of Him is the most wonderful existence. Paramahansa Yogananda, Man's Eternal Quest, P.46.
The "Songs of Songs" alludes to communion with God as His kiss: "Let him kiss me with the kisses of his mouth" (1:2,3). This is the state the Hasidim called yihud, "union," and ultimately, ayin, "nothing." Miriam Bokser Caravella, The Holy Name. P.261.
Abba Moses replied: 'To look upon God at all times and to be inseparable from Him, in the manner which you envisage, is impossible for a man still in the flesh and enslaved to weakness. In another way, however, it is possible to look upon God, for the manner of contemplating God may be conceived and understood in many ways. God is not only to be known in His blessed and incomprehensible being, (...). He is also to be known from the grandeur and beauty of His creatures, from His providence which governs the world day by day, from His righteousness and from the wonders which He shows to His saints in each generation. (...) There are many similar ways of seeing and apprehending God, which grow in us according to our labour and to the degree of our purification.' St. John Cassian (360-435), quoted in: The Philokalia, Vol. I., P.96-7.
A proud and spirited horse steps out delightedly once the rider is in the saddle. But the delighted intellect delights in the light of the Lord when, free from concepts, it enters into the dawn of spiritual knowledge. By continually denying itself, it advances from the wisdom necessary for the practice of the virtues to an ineffable vision in which it contemplates holy and ineffable things. Then the heart is filled with perceptions of infinite and divine realities and sees the God of gods in its own depths, so far as this is possible. Astounded, the intellect lovingly glorifies God, the Seer and the Seen, and the Saviour of those who contemplate Him in this way. St. Hesyhios the Priest (8th or 9th Century), quoted in: The Philokalia, Vol. I., P.185.
(...) where there is emptiness, ignorance is also to be found, but where there is richness of Spirit, no speech is possible. At such a time the soul is drunk with the love of God and, with voice silent, delights in His glory. St. Diadochos of Photiki (circa 400-486 CE), quoted in Philokalia, Vol. I., P.254.
It is for others to serve God, it is for you to cling to him; it is for others to believe in God, know him, love him, and revere him; it is for you to taste him, understand him, be intimate with him, enjoy him. William of Saint Thierry, Golden Epistle 16, quoted in: Pennington, Basil. (1978). O Holy Mountain! Journal of a Retreat on Mount Athos. P.86.
My understanding used to be like a stream that easily described all along the banks as its ken moved through the world. When I entered God, my vision became like His, it flooded out over existence, I knew no limits. The future I can now see with as much certainty as the past. If I stretched my arm its full length I could caress any creature in this universe; and Rabia does not exaggerate. Thus going to bed one night I knew a thief would be breaking in at 3 A.M., so I wrote a note and put it on my door, that said, “Could you wait till 4?” for the passions in prayers usually starts to wane by then. Rabia (c.717-801), quoted in: Ladinsky Daniel (2002). Love Poems from God. Twelve Sacred Voices from the East and West. P.25.
Mysticism in general