We'd only be lost if we'd ceased to fight for our liberty.
Prager and the religious right want to deny homosexuals the liberty of marrying the person they love. The Left (as Prager capitalizes it) is currently fighting to expand liberty on this issue, just as it previously fought to free and then to enfranchise African slaves, and to enfranchise women and non-propertied men. The word "liberal" (which Prager seems to equate to "the Left" at numerous points in his book) is derived from the same root as "liberty".
The idea that "liberty" belongs exclusively to "Americanism" (as Prager terms it) is nonsense.