Like thickened wine: summer's blood was in it Leaving stains upon our tongue (lines 6-7)
These lines recall the Eucharist (which represents the Last Supper), and the process of taking communion (on the tongue) during Catholic Mass. Heaney is connecting the earthly activity of eating berries to being in communion with Christ. See our discussion in "Symbols, Imagery, Wordplay" for more details.
Our hands were peppered With thorn pricks, (lines 15-16)
Thorns in the palms = crucifixion symbol. Check out our discussion in "Symbols, Imagery, Wordplay" for more details.
Each year I hoped they'd keep, knew they would not. (line 24)
Religion is built on hope. Even thought he's disappointed, the speaker's hope never fades or changes. "Each year," he says. Now that's some serious faith. It's the other half of the line that complicates the Christian idea of faith: the knowledge that no amount of faith will ever stave off disappointment. Heaney seems to be posing the question: If hope only brings a letdown, then what's the point?