You are quite correct that the church did not begin until the day of Pentecost.  But, I believe that in this passage which you cite, the Lord Jesus is looking forward to the church, and the principles of Christian conduct there, when the NT church shall be established, after His ascension into Heaven. For my rationale, let’s look at the verse you cite and those which come just after, “And if he shall neglect to hear them, tell it unto the church: but if he neglect to hear the church, let him be unto thee as an heathen man and a publican.  Verily I say unto you, Whatsoever ye shall bind on earth shall be bound in heaven: and whatsoever ye shall loose on earth shall be loosed in heaven.  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.  For where two or three are gathered together in my name, there am I in the midst of them.”

I will cite a portion of the Notes on Matthew written by J. N. Darby to help explain these Scriptures: “Here we are entirely upon new ground. It is not a question of Jehovah’s patience in grace with His people on the earth, but of the conduct of those who have part in the new privileges which flow from the new position taken by the Son of man. Important principles are also brought out. Authority resides in the assembly, the authority to bind and to loose. The true apostolic succession is in the two or three met in the name of Jesus. It is not in individual successors, either of Peter or of the other apostles, but in the assembly, that is found the spiritual authority sanctioned by heaven. Let the wisdom of an apostle, if there is one, guide them: it is none the less the assembly which judges as a last resource. It is the assembly that must be listened to. In it is found judicial authority – the power of binding and of loosing; and the reason for this is given, namely, that, where two or three are gathered to the name of Christ, He Himself is there. The same principle applies to the requests one presents to God. Where two or three agree to ask a thing, it is granted. It is not individual will, nor a purely personal desire. The two or three being gathered to the name of Jesus, Jesus is there. The request is the fruit of a spiritual agreement, and God answers the request.”

My dear friend, I think you can see from the notes above that what the Lord was outlining for His disciples could not have been experienced in the synagogue. While it is quite true that the Greek word used for “church” could refer to the “assembly” in the synagogue, verses 18-20 would not support this idea since we are clearly speaking of a gathering unto the name and Person of the Lord Jesus. The disciples might not have understood the concept of the New Testament church, but they would remember and be guided by His teaching after the Lord went back to Heaven and they were suffered to remain on the earth.  The assembly of believers would be those 2 or 3 (or more) gathered to the Lord, with Him in their midst. And as to verse 18, it would seem clear to me that only the assembly of believers (the NT church) would have the authority to make decisions and conduct actions relating to discipline in the assembly, those decisions being bound in heaven. I realize that this can be a bit confusing.

In summary, I believe that in Matthew 18:17, the Lord Jesus is indeed speaking of the New Testament church which would begin at Pentecost.  Thus the statements about discipline in Matthew 18 do very well correspond to our Scriptural guidance on assembly discipline in the local church, even though when stated by the Lord Jesus, the NT church had not yet begun.  (SF)  (587.4)