Liberal religion has a different conception of the authority than his question seems to assume. Traditionally, the purpose of worship was to present the scripture which was the source of authority. The synagogue cycle of readings, and then the lectionary of the Christian churches were all designed to teach the scripture to the people over the long course of multiple years. The preaching was to help people learn the scripture by applying it to contemporary situations and explaining its application and relevance. Scripture was the source of the authority.
Our religious tradition places the individual as the highest authority, but in the context of the covenanted community and in relationship to the minister called by that congregation for the purposes of spiritual leadership. The purpose of worship is the voluntary worship by the free soul , not the enlistment of people in the recognition of the authority of the Scripture. And so, the text/sermon has a different purpose in the liberal congregation.
The liturgy of the liberal congregation arises out of the long dialogue between congregation and its minister. In some cases, the liturgy of the church, the range of texts considered appropriate and essential to worship, the content of preaching, and the methods and purposes of prayers, have combined into a stable and enduring worship tradition which is the living theology and theological identity for that congregation.
In such a situation, the preacher proposes texts as having spiritual truth and authority each week in the liturgy. Of course, not every proposed text is a serious proposal. Some texts are chosen because they provide some factual content, or are funny, or set up the sermon well. Aside from those cases, the preaching says with each reading: "this text represents a truth we share, or one we have forgotten that we share, or a truth that I propose flows directly from other truths we share, even though we wish to avoid it." The authority to propose that text comes from the congregation.
If well chosen, that text becomes part of the common life of the congregation. It may bring a new note into worship; it may make room for a different kind of longing to be expressed.
We build and maintain buildings. We form and sustain communities. We are also enriching, developing and deepening the worship tradition of the congregations we serve. It is the most important way that we do serve the people in those congregations: we help them find a way authentic to them, and inclusive of their whole lives, in which they are able to worship.