The most gracious and merciful God, my brethren, is one and anyone who says that there are many gods is a devil. He is also a Trinity: Father, Son, and Holy Spirit, one nature, one glory, one kingdom, one God. He is incomprehensible, an inscrutable Lord, omnipotent, all light, all joy, all compassion, all love. We have no example to compare with the Holy Trinity because there is none in the world. But so that our minds can get some small help, the theologians of the Church give us a few examples. Among others, they use the sun. We all know that the sun is one as God is one also. And just as the sun illuminates this perceptible world, so does the Holy Trinity, God, illumine the intelligible world.
We have said, my brethren, that the sun is one but it is three at the same time. It has rays which come to our eyes like lines, like threads. It also has light which spreads throughout the entire world. We liken the eternal Father with the sun, the co-eternal Son with the rays, and the consubstantial Spirit with the light.
There is still another way in which you can understand the Holy Trinity. How? Confess candidly, receive the holy Sacraments with fear and reverence, and then the grace of the Holy Spirit will enlighten you so that you will understand better. It is this Holy Trinity that we pious and Orthodox Christians glorify and worship. He is the true God, and all others except for the Holy Trinity who are called god, are demons. And it is not only we who believe, glorify, and worship the Holy Trinity, but prophets, apostles, martyrs, ascetics, as numerous as the sand of the sea and the stars of the sky, who have shed their blood for the love of the Holy Trinity and have attained paradise and rejoice forever.
Similarly, men and women have denied the world, have gone off into the deserts and practiced asceticism all their lives and have gone to paradise. In addition, men and women lived in the world with prudence and virginity, with fasting, prayers, alms, and good works, and lived well here on earth and went to paradise to rejoice forever.
There is no place from where God is absent. We pious Christians should consider that God is in our hearts when we wish to commit some sin, that he is present everywhere, and that he sees us. We should be ashamed before the angels, the saints, and especially before the angel who guards our soul and observes us. We are embarrassed before a young child when we commit a sin, so how can we not be embarrassed before so many saints and angels?
– From Father Kosmas the Apostle of the Poor: The Life of St. Kosmas Aitolos Together With an English Translation of His Teaching and Letters by Nomikos Michael Vaporis, Brookline, MA: Holy Cross Orthodox Press, 1977.