Till he come (αχρ ου ελθη). Common idiom (with or without αν) with the aorist subjunctive for future time (Robertson, Grammar, p. 975). In Luke 22:18 we have εως ου ελθη. The Lord's Supper is the great preacher (καταγγελλετε) of the death of Christ till his second coming (Matthew 26:29).