“They who stand ready to content Love are also eternal and unfathomable. For their conversation is in heaven, and their souls follow everywhere their Beloved who is unfathomable”

Variant: Love involves a peculiar unfathomable combination of understanding and misunderstanding.

"To Shakespeare"
Poems (1851)
Context: The soul of man is larger than the sky,
Deeper than ocean, or the abysmal dark
Of the unfathomed center. Like that ark,
Which in its sacred hold uplifted high,
O'er the drowned hills, the human family,
And stock reserved of every living kind,
So, in the compass of the single mind,
The seeds and pregnant forms in essence lie,
That make all worlds. Great poet, 'twas thy art
To know thyself, and in thyself to be
Whate'er Love, Hate, Ambition, Destiny,
Or the firm, fatal purpose of the Heart
Can make of Man. Yet thou wert still the same,
Serene of thought, unhurt by thy own flame.

Source: The Doctrine of the Mean

"The Reaction in Germany" (1842)
Often paraphrased as, "The urge to destroy is also a creative urge"
Context: We exhort the compromisers to open their hearts to truth, to free themselves of their wretched and blind circumspection, of their intellectual arrogance, and of the servile fear which dries up their souls and paralyzes their movements.
Let us therefore trust the eternal Spirit which destroys and annihilates only because it is the unfathomable and eternal source of all life. The passion for destruction is a creative passion, too!

This I Believe, p. 232
Vokes - My Story (1985)

Trees and Other Poems (1914), Delicatessen
Context: For, once he thrilled with high romance
And tuned to love his eager voice.
Like any cavalier of France
He wooed the maiden of his choice.
And now deep in his weary heart
Are sacred flames that whitely burn.
He has of Heaven's grace a part
Who loves, who is beloved in turn.

Letter to Queen Mother Elisabeth of Belgium (9 January 1939), asking for her help in getting an elderly cousin of his out of Germany and into Belgium. Quoted in Einstein on Peace edited by Otto Nathan and Heinz Norden (1960), p. 282
1930s
Context: The moral decline we are compelled to witness and the suffering it engenders are so oppressive that one cannot ignore them even for a moment. No matter how deeply one immerses oneself in work, a haunting feeling of inescapable tragedy persists. Still, there are moments when one feels free from one's own identification with human limitations and inadequacies. At such moments, one imagines that one stands on some spot of a small planet, gazing in amazement at the cold yet profoundly moving beauty of the eternal, the unfathomable: life and death flow into one, and there is neither evolution nor destiny; only being.

Saraswati, S. 2001. The true history and the relfigion of India: a concise encyclopedia of authentic hinduism. Motilal Banarsidass.

Vol. I, Luke VIII: 16–21, p. 257
Expository Thoughts on the Gospels: St. Luke (1858–1859)