“Christ speaks of two debtors, one of whom owed much and the other little, and who both found forgiveness. He asks: Which of these two ought to love more? The answer: The one who has forgiven much. When you love much, you are forgiven much-and when you are forgiven much, you love much. See here the blessed recurrence of salvation in love! P. 176”

1850s, Two Discourses at Friday Communion (August 1851)

Source: The Accidental Tourist

Source: Quotes from secondary sources, Dictionary of Burning Words of Brilliant Writers, 1895, P. 589.

Source: Vanishing Acts

Source: The Goldfinch

this is a line spoken by Frank Morgan's depiction of the Wizard of Oz in the 1939 film, which debuted 20 years after Baum's death. It did not actually appear in the "Wonderful Wizard of Oz". The ending of "Steam Engines of Oz" wrongly attributes this phrase to Baum when it would've originated from the 1939 adaptation script writers Langley/Ryerson/Woolf.
Misattributed
Variant: A heart is not judged by how much you love; but by how much you are loved by others
Source: The Wonderful Wizard of Oz

"To My__" (December 1890)

The Inferno (1917), Ch. XVI
Context: Turn where you will, everywhere, the man and the woman ever confronting each other, the man who loves a hundred times, the woman who has the power to love so much and to forget so much. I went on my way again. I came and went in the midst of the naked truth. I am not a man of peculiar and exceptional traits. I recognise myself in everybody. I have the same desires, the same longings as the ordinary human being. Like everybody else I am a copy of the truth spelled out in the Room, which is, "I am alone and I want what I have not and what I shall never have." It is by this need that people live, and by this need that people die.

"Canzone 137", as reported in Bartlett's Familiar Quotations (1968), p. 163
Il Canzoniere (c. 1351–1353), To Laura in Life