Peter's question probably came in response to Jesus' teaching about restoring repentant sinners to fellowship with God and others (Matthew 18:15-20).  Jesus had advocated honest but loving and gracious confrontation of sinful behaviour, encouraging repentance which would lead to reconciliation.  Peter understood what Jesus was saying, but then he asked how persistent offenders should be treated.
 
The rabbis of their day taught that forgiveness should be offered on three occasions only.  Peter sensed that Jesus might be more generous and asked if the limit of three offences might be extended to seven times (seven was considered to be the number of perfection or completeness).  In response, the Lord greatly multiplied the number Peter suggested – the Greek is probably better translated as seventy times seven.  In other words, Jesus was saying that there is no limit to forgiveness in God's economy: there is no limit to His grace (John 3:34) and no limit to the number of times that repentance must be met with love and reconciliation.
 
Peter's question seems to presume that he would always be right, and it would always be the fault of others which would be the problem!  He would soon learn that Jesus was willing to restore him after his denial (John 21:15-17).  God's love is very generous indeed (James 1:5).  Without that, there would be no gospel of grace.  Yet it is surprising how many people cower in their sins and try to hide from God, even though He longs for them to rejoice in His forgiveness.
 
God is not vengeful towards people who are repentant.  He keeps on loving them, drawing them back into relationship with Himself.  The church needs to learn to do the same (Luke 17:3-5).  Repentance must always be met by forgiveness and reconciliation.  Of course, there are plenty of sinners who do not believe they are in the wrong: they cannot enjoy the grace of forgiveness or joy of reconciliation.  But those who are truly sorry for their sins must be assured that Father God wants to restore them (1 John 1:9); and the church must also gladly welcome them into a renewed fellowship.