The interchange in Matt 18:21–22 looks back to Jesus’ immediately preceding comments on handling a community member (ἀδελφός) who sins (Matt 18:15–20; Chrysostom, Hom. Matt., 61.1 [NPNF1 10:357]; cf. Matt 18:21; 19:1). Read within this context, Peter’s question ποσάκις ἁμαρτήσει εἰς ἐμὲ ὁ ἀδελφός μου καὶ ἀφήσω αὐτῷ; (Matt 18:21a; How many times* shall my brother sin against me and I forgive him?) addresses a very plausible ambiguity in Jesus’ preceding comments. Judging from this question, Peter presumably thinks it inappropriate for a community member endlessly to sin and repent, but as long as some repentance was involved, Jesus’ instructions could seem never to allow further action to be taken. As many times as the community member would sin and repent, this member would also be restored (Matt 18:15b; Chrysostom, Hom. Matt., 61.1 [NPNF1 10:357]).
Other things may be in view also, but someone who might try to “work the system” could certainly fall within the range of Peter’s concern here. Doubtless, Peter’s ἕως ἑπτάκις; (Matt 18:21b; Up to seven times?) was, to him, a generous number of repetitions for this situation (cf. Lightfoot, Commentary, 2:259), but following on Jesus’ previous comments (Matt 18:15–20), Peter’s essential question remains quite understandable (Chrysostom, Hom. Matt., 61.1 [NPNF1 10:357]). Yet, Jesus’ response of ἑβδομηκοντάκις ἑπτά (Matt 18:22; “seventy-seven times” or “seventy times seven times”) expands Peter’s proposal to almost unimaginable proportions and certainly to ones impractical to count (Chrysostom, Hom. Matt., 61.1 [NPNF1 10:357–58]; Snodgrass, Stories, 67).
Despite questions about Matthew’s composition history at this point (see Blomberg, Parables, 240–41; Snodgrass, Stories, 67), if one reads with the narrative of Matt 18 as it stands, the immediately following parable of the unforgiving servant (Matt 18:23–35) can play a meaningful role in further responding to Peter’s question. In particular, the parable’s concluding interpretation urges the practice of forgiveness on the part of each community member in the face dire consequences if this instruction is not followed (Matt 18:35). In this connection, the parable develops Jesus’ earlier comment (Matt 18:22) and suggests that Peter’s inquiry about forgiveness is simply the wrong question. Forgiveness per se is an essential, constitutive feature of Jesus’ community (Augustine, Civ., 15.6 [NPNF1 2:287]), and it is the presupposition rather than the goal of the procedure that Jesus outlines in Matt 18:15–18, which focuses on the related but distinct issue of restoring damaged fellowship (see Matt 18:15b; cf. Phil 3:8; Augustine, Serm., 32.4 [NPNF1 6:358]). Thus, in Matthew’s narrative, rather than directly answering Peter’s question, Jesus highlights Peter’s misunderstanding in order to stress the importance of forgiveness as a prerequisite for how the community approaches resolving its own internal offenses when they occur (Augustine, Serm., 32.4, 7 [NPNF1 6:358–59]).
* See BDAG, s.v. ποσάκις; Chrysostom, Hom. Matt., 61.1 (NPNF1 10:357). Rather than the common rendering “how oft(en)” (e.g., ESV, KJV, NASB95, NRSV, RSV) which focuses on the frequency of an offense, Peter’s immediately following question (ἕως ἑπτάκις; Matt 18:21b; Up to seven times?) suggests that ποσάκις may best be taken in the sense “how many times,” which focuses on the number of individual instances of an offense(s) (e.g., HCSB, NIV 1984; NIV 2011).