What eye has not seen, and ear has not heard,
and what has not entered the human heart,
what God has prepared for those who love him.

2 Corinthians, Chapter 2,9

St. Sister Faustina about heaven

(Selected parts of the Diary of St. Faustina Kowalska)

Today I was in heaven, in spirit, and I saw its inconceivable beauties and the happiness that awaits us after death. I saw how all creatures give ceaseless praise and glory to God. I saw how great is happiness in god, which spreads to all creatures, making them happy; and then all the glory and praise which springs from this happiness returns to its source; and they enter into the depths of God, contemplating the inner life of God, the Father, the Son, and the Holy Spirit, whom they will never comprehend or fathom. This source of happiness is unchanging in its essence, but it is always new, gushing forth happiness for all creatures. Now I understand Saint Paul, who said, “Eye has not seen, nor has ear heard, nor has it entered into the heart of man what God has prepared for those who love Him” (Diary 777).

Although the chosen ones in heaven see God face to face and are completely and absolutely happy, still their knowledge of God is not the same. God has given me to understand this. This deeper knowledge begins here on earth, depending on the grace [given], but to a great extent it also depends on our faithfulness to that grace (Diary 771).

And God has given me to understand that there is but one thing that is of infinite value in His eyes, and that is love of God; love, love and once again, love; and nothing can compare with a single act of pure love of God. Oh, with what inconceivable favors God gifts a soul that loves Him sincerely! Oh, how happy is the soul who already here on earth enjoys His special favors! And of such are the little and humble souls (Diary 778).

The sight of this great majesty of God, which I came to understand more profoundly and which is worshiped by the heavenly spirits according to their degree of grace and the hierarchies into which they are divided, did not cause my soul to be stricken with terror or fear; no, no, not at all! My soul was filled with peace and love, and the more I come to know the greatness of God, the more joyful I become that He is as He is. And I rejoice immensely in His greatness and am delighted that I am so little because, since I am little He carries me in His arms and holds me close to His Heart (Diary 779).

Oh, how ugly the earth when one knows heaven!(Diary 899).. When, during adoration, I repeated the prayer, “Holy God” several times, a vivid presence of God suddenly swept over me, and I was caught up in spirit before the majesty of God. I saw how the Angels and the Saints of the Lord give glory to God. The glory of God is so great that I dare not try to describe it, because I would not be able to do so, and souls might think that what I have written (8) is all there is. Saint Paul, I understand now why you did not want to describe heaven, but only said that eye has not seen, nor ear heard, nor has it entered into the heart of man what God has prepared for those who love Him [cf. I Cor. 2:9; 2 Cor. 12:1-7]. Yes, that is indeed so. And all that has come forth from God returns to Him in the same way and gives Him perfect glory (Diary 1604).