In Greek, the word translated impossible starts the sentence for emphasis. It means this absolutely cannot happen (see also Hebrews 6:18; 10:4; 11:6).
They were once enlightened when they "first learned about Christ" (see 10:32 and corresponding study note).
The good things of heaven might refer to the manna given from heaven (Exodus 16:4, 15; Psalm 78:24) as a symbol of spiritual blessings.