Henry Wei in his book "The Guiding Light of Lao Tzu", Synergy Books International, speaks about this mystic union.
"The mystic union represents the unique experience stemming from "the flight of the alone to the Alone" as the famous mystic Plotinus has so well expressed. Unfortunately, it cannot be easily attained much less isolated and measured and experimented upon in the laboratory. Its actuality is entirely based on the experience and confession of the mystic themselves who were completely convinced of its authenticity. Some skeptics have maintained that the mystics are victims of hallucination or self-hypnosis."
Agathiyar told us the same too that people would not understand the mystic until they come to that state too. Many of the things Agathiyar and Tavayogi told me in the Nadi in the former and in passing in the latter only made sense now after gaining the experience too.
Henry Wei writes of those experiencing this mystic union: "Their scale of values changes radically; their attitude towards life changes too; their health improves; they have a sharper and deeper understanding and a clear spiritual vision. In all these, man is understood as being capable of attaining union with the mysterious cosmic influence or cosmic spirit or divine being."
G.Valmikanathan in his book ‘Makers of Indian Literature - Ramalingam’, published by Sahitya Akademi (e-book at http://www.vallalar.org) traces Ramalinga Adigal’s brilliant journey on the pathway to God dividing it into three major portions beginning with the purgative way, the illuminative way, and finally the unitive way. The journey on the unitive way is where we see results. The transformation takes place rapidly without our doing. The Divine gifts us a body that can sustain his majestic presence. He personally sees through the necessary transformation to become him. Ramalinga Adigal affirms that the most impure body will become a body pure and imperishable. "The Unitive Way is where God with name and form will disappear and where ultimately the worshiper and the worshiped, the seeker and the sought, the soul and the Godhead, will merge into each other."
Henry Wei quotes Rabindranath Tagore on this mystic union, "The Paramatman, the supreme soul, has himself chosen this soul of ours as his bride and the marriage has been completed."
"The Divinity descends and settles in the disciple's Heart, who becomes entirely filled by Him. Captivated by the "Ardent Fire" of the Divine Essence, the disciple stops thinking, feeling, acting, and even existing. He has become pure Love and Compassion, and this is what he spills on all the beings." (Source: http://www.ramalinga.com)
One crosses "the threshold of the Unitive Way where God with name and form will disappear and ultimately the worshiper and the worshiped, the seeker and the sought, the soul and the Godhead, will merge into each other. Our Swaamikal is now ready for the highest of all possible acts of grace, the crowning glory of the long years of thavam. (Source: G.Vanmikanathan in his "Pathway to God Trod by Saint Ramalingar", Bharatiya Vidya Bhavan, Bombay.)
"The Swaamikal was modesty incarnate. He did not like the limelight before which he was exposed to the world. After having gained all the Siddhis and attained the highest possible state, a man of such stature, Ramalinga Adigal chose to stay away from the limelight but the Lord acted otherwise. He (God) spoke to Raamalinga Swaamikal who was the chosen Son of God in the nineteenth century to establish a universal brotherhood of man freed of all shackling fetters of castes and creeds, of rich and poor, of black or white, a universal brotherhood born of a sense of oneness in the eyes of God and based on compassion, on compassion alone."