Why is the Holy Trinity Essential in Orthodox Spirituality?

This is a difficult but important question. The foundation of our spiritual life is based on the nature of God, One God in Three Persons, The Holy Trinity. So why is this truth about God so important in the Orthodox understanding of Theosis, our aim to seek union with God? How can our unity with God in eternity be guaranteed? Only if the divinity has also taken on flesh. The incarnation of God in Jesus Christ, His Son, shows the love God has for man. Why? Because for our union to be possible God cannot be some impersonal being. Because of the Triune nature of God, in our path to unity with God we will never lose our identity. It is because God has become man that we too will be assured of our identity in our eternal life. Like Christ related to the Father we too will have a personal relationship in heaven for eternity with Him nourished in His love. Since we know God to be trinitarian we know God can never be reduced to some infinite oneness but always exists as Three Persons. This is the true nature of God as found in Holy Scripture. The Orthodox Saints use the terms "life in Christ", Life in Spirit, and the "Spiritual life" to describe the nature of union with God.

The distinguishing feature of Christianity is this Triune nature of God. God became Man to transform us in Spirit. He communicates to us by the Spirit and His uncreated energies. God is never distant from us because His uncreated energies support our existence and nurture our spiritual growth.

Saint Simeon the New Theologian describes the union experienced by him as follows.

Even at night and in the midst of darkness I see, trembling, Christ opening the heaven for me and I behold how He himself beholds me from there and He sees me here and below together with the Father and the Spirit in the thrice holy light. Because this is one and the same nevertheless in three images, although it is only one. And it illuminates my soul brighter than the sun and floods my spirit covered with gloom.... And this miracle was even the more astonishing because it opened my eyes and helped me to see, and that which I saw is He himself. Because this light helped those who behold to know themselves in light of those who see in light see Him again. For they see the light of Spirit and in as much they see Him, they see the Son. Now he who has been made worthy to see the Son, sees also the Father.

Note how Saint Simeon sees God in distinct three persons. This is quite different from the way many western mystics like Eckhart see God. Eckhart only sees God as a unity of persons, one thing. While Simeon sees God in the differentiation of the three persons. Christians from the earliest times have seen Christ as the Son, something which is much greater than simply Christ the man. When we are blessed with divine insight in union with God we will find the living relationship between man and God like that which exists in the relationship among the Holy Trinity.

This relationship of the Three persons is also a demonstration of pure love of God. From this we know that God is truly love. Not only is He a God of love but our union with Him demands our love of Him.

Staniloae writes

Only a perfect community of supreme persons can nourish with its unending and perfect love, our thirst for love in relation to it and between ourselves. This relationship cannot be theory but must be lived too. This is so because love isn't satisfied with only being theory, but wants to give itself, to welcome and be welcomed...

The Trinity, radiated by this love which is proper to it, can't be lived and conceived without it uncreated energies in ever increasing levels. Love is characterized by this paradox. One the one hand it unites object who love each other, on the other hand, it doesn't confuse their identity...

God wants to gradually extend the gift of His infinite love to another order of conscious subjects and namely to created ones. He wants to extend this love in its paternal form as toward other sons united with His Son. 

So after the creation of man, He wanted His Son to become man so that His love for His Son, made man, would be a love which is directed toward any human face, like that of His Son. In the Son made flesh we are all adopted by the Father... God made man as an image of His Son so that His Son could become man too. The Father loves all of us in His Son, because the Son was made our brother....The Son's love for us isn't separated from the Father's love for us, but in His love as a brother He makes the Father's love and also His love for the Father, engulf us. In us the Father welcomes other loving and loved sons because His Son was made our beloved brother.

Also, with this love of the Father and our love for Him in Christ, love is poured on us in the form of the Holy Spirit, His uncreated energies.

Staniloae

If the Son had not become man we would not have received the love of the Father. It is in Christ as man that it reached us. It is by the incarnate Son that the Holy Spirit radiates within humanity and the world is the love of God for us of outs for God.

It is the Spirit that brings into creation Trinitarian love raising creation to the level of divine love and Theosis. This is why we invoke the Holy Spirit in all our sanctifying services. It is by the Holy Spirit that creation is raised up to the divine world and the divine realm penetrates us. With this we are changed. This is the nature of Orthodox spirituality. Our aim is to acquire his uncreated energy that comes though the love of God in the Holy Spirit. This much more than the understanding of God intellectual through the Scriptures. Man can be joined with God though His uncreated energies that we are blessed with by the Holy Spirit.

Reference: Orthodox Spirituality by Fr, Dumitru Staniloae, pp 46-55.