So if you have told someone the gospel, and they’ve responded by becoming Christian, you may already have accidentally planted a ‘church’. After all, it only takes two or three to constitute a church, and when it meets, Jesus is there in the midst (Matt 18:20). Now, having planted a church is not a cause for panic. Rather, praise God and keep at it! Keep meeting with your church and teaching him or her the Bible, as this is the only way God uses to ensure that it will endure to judgement day.
If this sounds simple, it is. The only equipment required for church planting is the gospel (supplied by God) and a mouth to speak it with. If you care to compare it with terrorism, it is considerably easier and cheaper to achieve, and it has more powerful results and longer lasting (i.e. eternal) consequences.
Extract from Firing up for evangelism