An idiom is an expression whose meaning cannot be derived from its words. "Pie in the sky" is such a phrase. You know what it means only from experiences having heard it used in certain contexts. It means that an idea or plan is so unrealistic or impractical that it will never materialize. It is an empty promise or impossible dream. Such a person has his "head in the clouds" meaning that he is a dreamer and out of touch with reality. The empty wish is "pie in the sky" meaning that it won't come to pass. There's a phrase criticizing a religious idealist who is "so heavenly preoccupied that he's no earthly good." In 1911 a labor organizer named Joe Hill wrote the phrase about pie in a song criticizing the Salvation Army song "In the Sweet Bye and Bye." He was critical of preaching which contained promises of future rewards but did little to alleviate suffering in this life. He wrote, "You'll get pie in the sky when you die." But "faith is the assurance of things hoped for, the conviction of things not seen" (Heb 11:1).
It is not false hope as in the idiom above. But there is a caveat. Proverbs 13:12 says "hope deferred makes the heart sick." When you become depressed it's possible to lose faith if your expectation vanishes. Nonetheless, "'Though He slay me, I will hope in Him'" (Job 13:15). Don't assume you must accomplish something unilaterally as Israel "presumed to go up to the hill country" (Nu 14:44) and their enemies defeated them. Sarai assumed that "the Lord has prevented me from bearing children" (Ge 16:6) and it resulted in Ishmael being born (:15). At the right time the Lord said "Sarah your wife shall have a son" (18:10) even though she was "past childbearing" (:11). Therefore cast "all your anxiety on Him, because He cares for you" (1Pe 5:7). Moses' solution is to "love the Lord your God with all your heart and with all your soul . . . [and] follow the Lord your God and fear Him . . . [and] listen to His voice, serve Him, and cling to Him" (Dt 13:3-4). It is "'Not by might nor by power, but by My Spirit,' says the Lord of hosts" (Zec 4:6).