It would be that it is the Word of God that changes lives and grows the church. I could tell you this because I have seen it over a long time working in a variety of congregations in four quite diverse dioceses.
But that would be anecdotal evidence. Or perhaps testimony. Notoriously unreliable some would say.
I suppose I came to this belief from two foundations a long time ago. One foundation was my own experience of growing to know Christ. I know I grew as I read and heard the scriptures expounded. The other foundation was the scriptures themselves. They are not merely religious history, or a kind of Christian life manual. They tell us what God has done and said. And it is clear that a great deal of what he has done is a result of what he has said.
By his word he created the heavens and the earth. He spoke to Abraham and made promises which he fulfilled because he had said he would. He spoke through prophets and fulfilled those promises through his Son.
When Jesus was raised from the dead and was declared to have all authority in heaven and on earth he sent his disciples to the nations with a message of repentance and forgiveness. Amazingly this message, so despised by so many, has been the means of turning millions and millions to the Lord Jesus.
The gospel message spoken by disciples and empowered by the Spirit is what changes people and saves them. Some want short cuts. Techniques. Methods.
But the time of the greatest expansion of the message (the century after Pentecost) none of these short cuts was used. In fact they were rejected. In favour of the power of the gospel itself.
One of the reasons for the poverty and weakness of the church is its neglect and patronising of the Word of God.
There is one thing that changes lives and churches, it is God's word, taken by his Spirit into the hearts and minds of his people by their own reading, and by their being exposed to it by the equippers God has supplied.
Dale