Matthew 18:19 - 20, "Again I say to you, that if two of you shall agree on the earth concerning any matter, whatsoever it may be that they shall ask, it shall come to them from my Father who is in the heavens. For where two or three are gathered together unto my name, there am I in the midst of them."
Acts 2:1 - 2 we read a very famous account of corporate prayer and its powerful result, "And when the day of Pentecost was fully come, they were all with one accord in one place. And suddenly there came a sound from heaven as of a rushing mighty wind, and it filled all the house where they were sitting."
Acts 4:24 - 32 gives another powerful account of how effective corporate prayer is.
Ephesians 6:18: "Pray at all times - on every occasion, in every season - in the Spirit, with all [manner of] prayer and entreaty ..."
