If I Forget Thee, O Jerusalem...!

For us in these modern days, the following Psalm has a symbolic meaning. The Fathers of the Holy Mother Church say that Jerusalem is HOME, Jerusalem is all the fine, beautiful, grace-filled, innocent, marvelous, holy, divine, Heavenly things that we must cling to in our hearts. Not just New Jerusalem, which is the Kingdom of Heaven, but all the things that are within that kingdom, all the things that are outside that Kingdom trying to get in, all the things that strain their faces toward the divine sunlight that emits from those gates.
In The Brothers Karamazov, one grieving father used it as a lament for his dead son. He said "Oh Jerusalem, Jerusalem, if I forget thee...". The son had already entered that Kingdom which was already in full glory in his heart.
Here is the full Psalm:

Psalm 137 King James Version

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1By the rivers of Babylon, there we sat down, yea, we wept, when we remembered Zion.

2We hanged our harps upon the willows in the midst thereof.

3For there they that carried us away captive required of us a song; and they that wasted us required of us mirth, saying, Sing us one of the songs of Zion.

4How shall we sing the LORD'S song in a strange land?

5If I forget thee, O Jerusalem, let my right hand forget her cunning.

6If I do not remember thee, let my tongue cleave to the roof of my mouth; if I prefer not Jerusalem above my chief joy.

7Remember, O LORD, the children of Edom in the day of Jerusalem; who said, Rase it, rase it, even to the foundation thereof.

8O daughter of Babylon, who art to be destroyed; happy shall he be, that rewardeth thee as thou hast served us.

9Happy shall he be, that taketh and dasheth thy little ones against the stones.

Earlier in the story:

"What are you weeping for?"

"It's my little son I'm grieving for, Father. he was three years old- three years all but three months. For my little boy, Father, I'm in anguish, for my little boy. He was the last one left. We had four, my Nikita and I, and now we've no children, our dear ones have all gone I buried the first three without grieving overmuch, and now I have buried the last I can't forget him. He seems always standing before me. He never leaves me. He has withered my heart. I look at his little clothes, his little shirt, his little boots, and I wail. I lay out all that is left of him, all his little things. I look at them and wail. I say to Nikita, my husband, 'let me go on a pilgrimage, master.' He is a driver. We're not poor people, Father, not poor; he drives our own horse. It's all our own, the horse and the carriage. And what good is it all to us now? My Nikita has begun drinking while I am away. He's sure to. It used to be so before. As soon as I turn my back he gives way to it. But now I don't think about him. It's three months since I left home. I've forgotten him. I've forgotten everything. I don't want to remember. And what would our life be now together? I've done with him, I've done. I've done with them all. I don't care to look upon my house and my goods. I don't care to see anything at all!"

"Listen, mother," said the elder. "Once in olden times a holy saint saw in the Temple a mother like you weeping for her little one, her only one, whom God had taken. 'Knowest thou not,' said the saint to her, 'how bold these little ones are before the throne of God? Verily there are none bolder than they in the Kingdom of Heaven. "Thou didst give us life, O Lord," they say, "and scarcely had we looked upon it when Thou didst take it back again." And so boldly they ask and ask again that God gives them at once the rank of angels. Therefore,' said the saint, 'thou, too, O Mother, rejoice and weep not, for thy little son is with the Lord in the fellowship of the angels.'