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The problem is the overwhelming sense of being in the right many people have when they believe they are in possession of God's truth.
This is understandable but enthusiasm for a single tradition betrays lack of faith (illustrated). Let us consider faith for a moment. No-one ever makes an enthusiastic response to the undoubted fact the sun will rise tomorrow. It's true, there's no denying it. But I don't need to persuade anyone of the fact.
However, where I'm not persuaded myself, I have to be enthusiastic. The claims I make for the truth of my beliefs compensate for my own doubts.
As confidence grows, so enthusiasm falls away. I can have confidence in the faith into which I have been formed within my own tradition. This is why formation is so important. Without formation, I will not grow in confidence and so remain absolute in my beliefs.
Christopher Jamieson writes in his book, 'Finding Sanctuary', about humility. Humility is not denial of truth but demonstrates growing confidence in it. It means I can listen to others without feeling threatened or getting angry. I find as I listen to others' views, I grow in faith within my own tradition.
Perhaps a weakness of mission is its failure to form its converts? If no tradition is offered, people have nothing to argue with, to learn from, to grow with. We cannot be reconciled with other traditions if we have no tradition of our own. We'll either be overwhelmed by the confidence of the other or seek to overwhelm them.
As the roots of our faith grow deeper into our own tradition, we are able to walk alongside members of other traditions, learn from them and possibly have something of value to pass onto them.