Jesus Verse by Verse...
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18:3 Conversion has various levels. The disciples  had been converted, but there was a higher level of conversion- to become like  children. Baptism isn’t the end of our conversion- it’s part of the process. 
  18:5 Jesus invites us to see Him as represented by  the child. In the 1st Century world, children and women were  considered as non-persons. But Jesus paid great attention to them and showed  how He valued them as persons.
  18:6 Such drowning in the sea as a millstone is  Babylon’s judgment (Rev. 18:21). Those who make others sin or stumble will  share in Babylon’s punishment at the last day- they will be “condemned with the  world” (1 Cor. 11:32). Those rejected by Jesus will be sent back into the world  they so loved- and share its judgment.
  18:8 Everlasting fire is symbolic of total  destruction. Jerusalem was punished with this (Jer. 17:27)- but there is no  eternal fire there today. Don’t read ‘eternal fire’ literally.
  18:17 Unto  you- you singular [Gk.]. This passage speaks about personal disagreements.  If you follow this process, the end result is that you personally will treat  the other person as a Gentile and tax collector- both of which groups Jesus  especially sought to minister to and win for Him.  It doesn’t speak about a church  disfellowshipping an individual.
  18:20 Jesus is with us all the time individually,  but He is especially with us when we gather together. Hence the importance of  believers meeting together rather than believing in isolation.
  18:21 Sin  against- Peter refers back to v. 15. Jesus seems to be saying that we can  go through the process He outlines in :15-17. But the higher level is to  forgive unconditionally. If somebody sins against us 490 times / day and claims  each time to have repented, it’s clear their repentance is insincere. Jesus is  saying we should forgive people without trying to assess nor believe the  sincerity of their repentance. For the nature of the forgiveness we show others  is what we will be shown. 
  18:24 There’s an element of unreality in this  story. How could a person amass such a huge debt? Only because the King kept on  and on lending to him, knowing the man could never repay. This speaks of God’s  forgiveness of us and the enormity of our debt to Him. Realizing our sinfulness  will motivate us to forgive others. 
  18:31 When we see unreasonable behaviour from our  brethren, the response is to tell it to the Lord. He knows about it already,  but it’s good for us psychologically to tell the story to Him in prayer.