This might mean the exact opposite of how you’ve heard it used:
“Where two or three are gathered in my name, there am I among them.” (Matthew 18:20)
This is not about casual, ad hoc churches that, like vapors, emerge briefly and then disappear. Neither is it about the mere universal church that spans the globe, inclusive of all Christians, expressed in unplanned intersections of believers at a coffee shop.
The context (18:15-20) is more serious. It describes a protocol of increasing escalation of confrontation that eventually arrives, if necessary, at excommunication. Let’s read it:
“If your brother sins against you, go and tell him his fault, between you and him alone. If he listens to you, you have gained your brother. But if he does not listen, take one or two others along with you, that every charge may be established by the evidence of two or three witnesses. If he refuses to listen to them, tell it to the church. And if he refuses to listen even to the church, let him be to you as a Gentile and a tax collector. Truly, I say to you, whatever you bind on earth shall be bound in heaven, and whatever you loose on earth shall be loosed in heaven. Again I say to you, if two of you agree on earth about anything they ask, it will be done for them by my Father in heaven. For where two or three are gathered in my name, there am I among them.”
It refers to identifiable, regular, governed gatherings that are organized enough to recognize leadership and implement, in a church-wide coordinated way, church discipline.
The verse is about Jesus putting his divine stamp of approval, as though bodily present, on a properly administered act of church discipline.
Paul uses similar language referring to church discipline:
“For though absent in body, I am present in spirit; and as if present, I have already pronounced judgment on the one who did such a thing. When you are assembled in the name of the Lord Jesus and my spirit is present, with the power of our Lord Jesus, you are to deliver this man to Satan for the destruction of the flesh, so that his spirit may be saved in the day of the Lord.” (1 Corinthians 5:3-5)
It’s the kind of thing that would be exercised on “anyone who bears the name of brother if he is guilty of sexual immorality or greed, or is an idolater, reviler, drunkard, or swindler.” (1 Corinthians 5:11)
Or someone who, though having called themselves a brother, has (via persistent bad behavior or undue absence) separated themselves from the local church they were once committed to.
So if you’re inclined to think that the passage means you don’t need a well-regulated church, submission to elders, church discipline (e.g. excommunication), or a regular, identifiable gathering, or some semblance of church membership (at least defined as identifiable, mutually affirmed recognition of Christian faith and belonging), then you’re taking the text in the exact opposite direction.
Churches are outpost-embassies of a kingdom that are marked by authority, governance, corporate unity, and regular gatherings. If you find yourself needing restoration from sin, but refuse the gentle (and then increasingly firm) pleadings of your local church, you just might find your name announced at a church members meeting.
And if you have refused to attach yourself to a local church, it’s as though you’ve preemptively excommunicated yourself from God’s people.
Sober up and fear God. That act of discipline (Paul favorably called it “judging” and “purging” in 1 Corinthians 5:13), rightly administered, is given as though Jesus sits bodily on his throne at the local church. “Where two or three are gathered in my name, there am I among them.” (Matthew 18:20)