The feast of the Holy Trinity (otherwise, Pentecost) is dedicated to the descent of the Holy Spirit upon the Apostles on the fiftieth day after the Resurrection of Christ. This event brought into existence the Church of Christ and gave rise to the Christian faith on earth. On the feast of Pentecost the Church brings its children to the doors of its spiritual life and appeals to them to renew and strengthen in themselves the gifts of the Holy Spirit given to them in Baptism. Spiritual life of an individual is impossible without God's grace, which possesses the mysterious power of rebirth and transforms the whole of the Christian's inward life. However lofty and valuable his desire might be, it will be fulfilled by the Holy Spirit. That is why the feast of Pentecost is always so joyfully celebrated by Orthodox Christians.

Throughout all of history God gradually revealed himself to mankind. During Old Testament times people knew only about God the Father. Since the birth of the Savior they learned of his Only-begotten Son, and on the day of the descent of the Holy Spirit people learned of the existence of the Third Person of the Holy Trinity. Thus mankind was instructed to believe and praise God, one in essence and Threefold in Persons, that is, God the Father, and the Son, and the Holy Spirit, the Trinity one and indivisible.

Bishop Alexander (Mileant), http://www.fatheralexander.org/booklets/english/pente.htm