Again I say unto you - The word αμην, verily, is added here, in ninety-eight MSS., (many of which are of the greatest antiquity and importance), seven editions, all the Arabic, the Slavonic, and several of the Itala. The taking in or leaving out such a word may appear to some a matter of indifference; but, as I am fully convinced Jesus Christ never spoke a useless or a needless word, my maxim is, to omit not one syllable that I am convinced (from such authority as the above) he has ever used, and to take in nothing that he did not speak. It makes the passage much more emphatic - Again, Verily I say unto you,

If two of you shall agree - ΣυμφωνηϚωσιν, symphonize, or harmonize. It is a metaphor taken from a number of musical instruments set to the same key, and playing the same tune: here, it means a perfect agreement of the hearts, desires, wishes, and voices, of two or more persons praying to God. It also intimates that as a number of musical instruments, skilfully played, in a good concert, are pleasing to the ears of men, so a number of persons united together in warm, earnest, cordial prayer, is highly pleasing in the sight and ears of the Lord. Now this conjoint prayer refers, in all probability, to the binding and loosing in the preceding verse; and thus we see what power faithful prayer has with God!

It shall be done for them - What an encouragement to pray! even to two, if there be no more disposed to join in this heavenly work.

That if.

Matthew 5:24 Leave there your gift before the altar, and go your way; first be reconciled to your brother, and then come and offer your gift.

Matthew 21:22 And all things, whatever you shall ask in prayer, believing, you shall receive.

Mark 11:24 Therefore I say to you, What things soever you desire, when you pray, believe that you receive them, and you shall have them.

John 15:7,16 If you abide in me, and my words abide in you, you shall ask what you will, and it shall be done to you...

Acts 1:14 These all continued with one accord in prayer and supplication, with the women, and Mary the mother of Jesus, and with his brothers.

Acts 2:1,2 And when the day of Pentecost was fully come, they were all with one accord in one place...

Acts 4:24-31 And when they heard that, they lifted up their voice to God with one accord, and said, Lord, you are God, which have made heaven...

Acts 6:4 But we will give ourselves continually to prayer, and to the ministry of the word.

Acts 12:5 Peter therefore was kept in prison: but prayer was made without ceasing of the church to God for him.

Ephesians 6:18-20 Praying always with all prayer and supplication in the Spirit...

Philippians 1:19 For I know that this shall turn to my salvation through your prayer, and the supply of the Spirit of Jesus Christ,

James 5:14-16 Is any sick among you? let him call for the elders of the church; and let them pray over him...

1 John 3:22 And whatever we ask, we receive of him, because we keep his commandments, and do those things that are pleasing in his sight.

1 John 5:14-16 And this is the confidence that we have in him, that, if we ask any thing according to his will, he hears us...

Revelation 11:4-6 These are the two olive trees, and the two candlesticks standing before the God of the earth...

it shall.

John 14:13,14 And whatever you shall ask in my name, that will I do, that the Father may be glorified in the Son...

John 16:23 And in that day you shall ask me nothing. Truly, truly, I say to you, Whatever you shall ask the Father in my name...

Forgiven and Unforgiving
'Jesus saith unto him, I say not unto thee, Until seven times; but, Until seventy times seven.' --MATT. xviii. 22. The disciples had been squabbling about pre-eminence in the kingdom which they thought was presently to appear. They had ventured to refer their selfish and ambitious dispute to Christ's arbitrament. He answered by telling them the qualifications of 'the greatest in the kingdom'--that they are to be humble like little children; that they are to be placable; that they are to use all means …
Alexander Maclaren—Expositions of Holy Scripture

The Wicked Servant
ST. MATTHEW xviii. 23. The kingdom of heaven is likened to a certain king, which would take account of his servants. This parable, which you heard in the Gospel for this day, you all know. And I doubt not that all you who know it, understand it well enough. It is so human and so humane; it is told with such simplicity, and yet with such force and brilliancy that--if one dare praise our Lord's words as we praise the words of men--all must see its meaning at once, though it speaks of a state of …
Charles Kingsley—The Water of Life and Other Sermons

Meetings for Prayer.
Text.--"Again I say unto you, That if two of you shall agree on earth as touching any thing that they shall ask, it shall be done for them of my Father which is in heaven."--Matthew xviii. 19. HITHERTO, in treating of the subject of Prayer, I have confined my remarks to secret prayer. I am now to speak of social prayer, or prayer offered in company, where two or more are united in praying. Such meetings have been common from the time of Christ, and even hundreds of years before. And it is probable …
Charles Grandison Finney—Lectures on Revivals of Religion