One of Virgil's most famous remarks is this: "Love conquers all." It's from the classic Eclogue 10, Gallus, and the somewhat larger context is this: "In hell, and earth, and seas, and heavens above, / Love conquers all; and we must yield to love." That final remark is very important, for love can only conquer all, if we yield to it. This sentiment finds a new expression in one of the most famous late-classical and early medieval works, Boethius's Consolation of Philosophy. This work from the sixth century AD is also, among other things, a work written in prison, where its author was falsely sent for a year (as I recall). The work has to do with Lady Philosophy who comes to console the prisoner (Boethius) to lift his despair.
What is amazing about this text is that its "message" has a lot to do with Thoreau's (though I doubt that Thoreau simply imitated it). Lady Philosophy consoles Boethius by telling him about the transitory nature of all earthly "goods," and by advocating the cultivation of the pleasures of the mind (This, in essence, is what Thoreau also endorses). The world's evils can be, thus, overcome by "virtue." At the heart of Lady Philosophy's message is the idea that it's love that makes the world go round. The relevant chapter has a song devoted to the idea that love is lord of all. The final lines of the song go like this:
Tribes and nations Love unites
By just treaty's sacred rites;
Wedlock's bonds he sanctifies
By affection's softest ties.
Love appointeth, as is due,
Faithful laws to comrades true—
Love, all-sovereign Love!—oh, then,
Ye are blest, ye sons of men,
If the love that rules the sky
In your hearts is throned on high!
What is also present in this song (and idea) is that it's "divine" love that we must - to use Virgil's words again - submit to. It's the love "that rules the sky" (divine love) that must come into our hearts. So love is the greatest good for us. Without it, we are nothing (as is also writ large in Paul's First Epistle to the Corinthians)
W. H. Auden included this same idea in the original version of one of his poems ("September 1, 1939"), where he gives expression to it this way: "We must love one another or die."