For one human being to love another: that is perhaps the hardest of all our tasks, the ultimate, the last test and proof, the work for which all other work is but preparation.

[Liebhaben von Mensch zu Mensch: das ist vielleicht das Schwerste, was uns aufgegeben ist, das Äußerste, die letzte Probe und Prüfung, die Arbeit, für die alle andere Arbeit nur Vorbereitung ist.]

Rainer Maria Rilke (1875-1963) German poet

Letter (1904-05-14) to Franz Xaver Kappus, Letters to a Young Poet [Briefe an einen jungen Dichter], No. 7 [tr. Norton (1934)]
    (Source)

(Source (German)). Other translations:

For one human being to love another human being: that is perhaps the most difficult task that has been entrusted to us, the ultimate task, the final test and proof, the work for which all other work is merely preparation.
[tr. Mitchell (1984)]

For one person to love another: that is perhaps the most difficult thing we are asked to do, the ultimate task, the final test and proof, the work for which all other work is mere preparation.
[tr. Searls (2020)]

For one human being to love another is perhaps the most difficult task of all, the epitome, the ultimate test. It is that striving for which all other striving is merely preparation.
[tr. Burnham]