Why 2 or 3? Why not 1?

Matthew 18:20 - "For where two or three come together in my name, there am I with them."

A well known verse. But why do you think it is two or three and not just one??

1 John 1:7 “But if we walk in the light as He is in the light, we have fellowship with one another, and the blood of Jesus Christ His Son cleanses us from all sin.”
“He who loves his brother abides in the light, and there is no cause for stumbling in him” (1 John 2:10)
“We know that we have passed from death unto life, because we love the brethren...” (1 John 3:14)
“...for he who does not love his brother whom he has seen, how can he love God whom he has not seen?” (1 John 4:20)
“...If we love one another, God abides in us, and His love has been perfected in us” (1 John 4:12)
“By this all will know that you are My disciples, if you have love for one another” (John 13:34-35)

If we refuse to gather with our brethren, we are refusing to gather with Jesus. Christianity is not a lone wolf religion. While it is true that to come into the "body of Christ" is by personal faith, we are not to live our lives apart from other believers. Look at how the church body functions in the book of Acts. We are to love one another, to edify one another, to be part of a local church body which was meant to resemble the larger universal body of believers. This is not to say that we cannot commune with God on a personal level, we certainly can. But when we fellowship with other believers, there is a special way that Christ is present.