HCSB Matt 18:20 For where two or three are gathered together in My name, I am there among them.I would like to caution against taking passages out of context. All my life I've heard this Matthew 18:20 reference cited to prove that "church" could be any grouping of Christians, down to the smallest number which might constitute a group.
But that's not what the passage is about; the passage is about the church's authority to make/apply rules in individual situations. In the Jewish Rabbinic culture in which Yahshua and his disciples were raised, the various religious authorities (this rabbi vs that rabbi; this school of thought vs that school of thought) would "bind" or "loose" various aspects of the Law of Moses depending on the situation.
For example, one rabbi would teach that if a donkey needed help on the Sabbath (doctoring, pulling out of a pit, etc) the needs of the donkey took precedence over the rules regarding doing no work on the Sabbath, whereas a different rabbi would insist that the donkey's needs must wait until the Sabbath had passed. Yahshua used this very issue to show that human needs take precedence over legal strictures, when he healed the man with the crippled hand on the Sabbath, which put him on the Pharisees' hit-list (Mark 3:1-6).
These various doctrinal "packages" from the various schools of thought / rabbis were called "yokes". Yahshua said his yoke was easy.
Most rabbis were average, run-of-the-mill rabbis, who could teach what had been determined by rabbis having authority, but they could not glean/apply "new" meaning from the text and teach it. They were limited to teaching what the community considered to be orthodox.
But a rare few rabbis were "s'mikeh rabbis", rabbis who had authority. (s'mikeh is pronounced something like "smee-hah!".) Within the previous generation, both Hillel and Gamaliel had been s'mikeh rabbis. When Yahshua came teaching as one having authority rather than like the scribes (Mark 1:22), it astonished his listeners. The officials then challenged him, asking how he had become a s'mikeh rabbi, and he answered them in typical Jewish fashion with his own question, asking them whether John the Immerser had s'mikeh. His implication was that he had gotten his authoritative position at least partially from John. (A s'mikeh rabbi had to be so endowed by two authorities: John was one; God (in the form of a dove descending on him post-immersion) was the other.) The officials refused to answer Yahshua's question, so he refused to answer theirs, again, in typical Jewish fashion.
In the context of Matthew 18:20, Yahshua is teaching his disciples that he is giving them s'mikeh, the authority to bind and loose. If two of these new s'mikeh rabbis agreed on anything, then that would be the "law" in the church. We later see an example of this in Acts 15, when the church elders and apostles gather together to decide the issue of what would be required of the new Gentile converts: do they have to convert to Judaism and be good Jews in order to be Christians, or not? (The decision was no, the Gentiles did not have to convert to Judaism in order to be Christians.)
For convenience, here's the context of the passage:
HCSB Matt 18:15 "If your brother sins against you, go and rebuke him in private. If he listens to you, you have won your brother. 16 But if he won't listen, take one or two more with you, so that by the testimony of two or three witnesses every fact may be established. 17 If he pays no attention to them, tell the church. But if he doesn't pay attention even to the church, let him be like an unbeliever and a tax collector to you. 18 I assure you: Whatever you bind on earth is already bound in heaven, and whatever you loose on earth is already loosed in heaven. 19 Again, I assure you: If two of you on earth agree about any matter that you pray for, it will be done for you by My Father in heaven. 20 For where two or three are gathered together in My name, I am there among them.