The tree begins with a single see. An acorn or its equivalent falls into the earth: tiny, vulnerable, alone. It germinates and puts out roots down into the dark earth. Simultaneously it send up a shoot into the light and air. The roots quickly diverge and probe all over the place, looking for nourishment and water. The shoot becomes a trunk, again a single upright stalk, but this, too, quickly diverges. An oak or a cedar will spread far and wide in all directions. Even the tall, narrow poplar is far more than just a single trunk. The river flows from many into one; the tree grows from one into many.
We need both images if we are to understand the church.
The church is like a river. In the last book of the Bible, John the visionary sees a huge throng of people from every nation, kindred, tribe, and tongue coming together in a great chorus of praise. Like the river, they all started in different places, but they have now brought their different streams into a single flow. The image of the river reminds us forcibly that, though the church consists by definition of people from the widest possible variety of backgrounds, part of the point of it is that they belong to one another, and are meant to be part of the same powerful flow, going now in the same single direction. Diversity gives way to unity.
But at the same time the church is like a tree. The single seed, Jesus himself, has been sown in the dark earth and has produced an amazing plant. Branches have set off in all directions, some pointing almost directly upward, some reaching down to the earth, some heading out over neighboring walls. Looking at the eager, outstretched branches, you'd hardly know they were all from the same stem. But they are. Unity generates diversity.
N.T. Wright-Simply Christian