Celtic Christianity: the form of Christian faith that flourished among the people of Ireland during the Middle Ages.  It has gained a great deal of attention lately. What many contemporary Christians do not realize is that Celtic Christianity was one of the most successfully evangelistic branches of the church in history. The Celtic church converted Ireland from paganism to Christianity in a remarkably short period, and then proceeded to send missionaries throughout Europe.  North America is today in the same situation as the environment in which the early Celtic preachers found their mission fields:  unfamiliar with the Christian message, yet spiritually seeking, and open to a vibrant new faith, yet occultly influenced. 

If we are to spread the gospel in this culture of secular seekers, we would do well to learn from the Celts. 
Their ability to work with the beliefs of those they evangelized, to adapt worship and church life to the indigenous patterns they encountered, remains unparalleled in Christian history. If we are to succeed in reaching the West ... again, then we must begin by learning from these powerful witnesses to the saving love of Jesus Christ.

Most people experience the faith through relationships.  They encounter the gospel through a community of faith and prayer.  Becoming a Christian involves a process that takes time.  "Christianity is more 'caught' than taught."  For most people, belonging comes before believing.

The Celts believed in the importance of the team.  A group of people can pray and think together.  They can inspire and encourage each other.  After all, we were called to be part of the Body of Christ, not Lone Ranger Christians.  In fact  we each aren't given all the gifts of the Holy Spirit to force each of us to depend upon the rest of the body.

They formed monastic communities that prepared people to live with depth, compassion and power in mission.  There were five experiences people needed.  One experience  was voluntary periods of solitude, where God could speak to you in the quietness.  A second experience was a one-on-one time with a peer with whom you were vulnerable and accountable.  They confessed their sins to one another and prayed for each other and encouraged each other and delivered each other from demons.  A third experience was time with a small group of ten or fewer brethren -- led by someone chosen primarily for their devotion.  A fourth experience was the common life.  The community ate together, worked together, learned together, prayed together and worshipped together.  A fifth experience was group ministry and witness to pre-Christian people.

The Celtic model of reaching people is different than the current traditional method.  The Celts first established community with people or brought them into the fellowship of their community of faith.  Within the fellowship, they engaged in conversation, ministry, prayer and worship.  In time, as the seeker discovers that they now believe, they would invite them to commit.  Thus their head is reached through their heart.

The current traditional approach is to have a trained expert present the Christian message.  The message is ended with an invitation to decide to believe in Christ and become Christians.  If they decide positively, welcome them into the church and its fellowship.  This is not often effective because it ignores the heart and the place of prevenient grace through prayer.  Often the person is left with little or no fellowship, to sink or swim on their own.  Discipling is an afterthought, even though Jesus commanded us to disciple new Christians.

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much of the above comes from "The Celtic Way of Evangelism" by George Hunter III who put it more succinctly than I could have.