To preface this post, this topic is specifically on what Biblical Theology is as a science, not as a reference to what is Biblical for there are biblical concepts which are not explicitly taught in scripture. This is a reference more specifically to what can be gained from the Bible directly.
As I read through the first chapter of the Moody Handbook of Theology, I was struck by how simplistic the definition and scope of Biblical Theology is. With the scope of Biblical Theology being limited to simply the theology that one can gain from the Bible alone without consulting external sources as one would do in a systematic theology. Biblical Theology is essentially the basic foundation from which all other theology and practical application can be obtained. I have to say that my favorite diagram was the diagram of Biblical Theology in relation to the other disciplines of Christian study. The diagram is not difficult to understand, but it is very accurate with how everything starts with scripture, and then through studying scripture we get our Biblical Theology which we can then use in order to get our system of theology which can then lend itself to practical application for ones life as well as apologetics in order to defend the theology.
If a church or denomination were to stray from biblical and then systematic theology, they would essentially be worshipping a God that is not the God of the Bible. This is also the case when people abandon accurate Biblical Theology and Systematic Theology and apply only sections of theology to their lives. It leads to worship of a God that is not really God but rather an idol that we have created of our own making. This is the path idolatry has taken once the worship of physical objects ceases. Once a church has started on a path that leads away from Biblical and Systematic Theology, there church will continue to stray further and further from truth ceasing only when someone steps up and gets back to the study of scripture and then Biblical Theology to get back to the roots of their belief and the worship of the true God.
This has not happened with my church or denomination, but I have seen it happen in many other denominations in America through the years, and throughout the world. It happens too often and one of the major signs tends to be a compromise with sin. This is not the only one, but when I see denominations accepting people that are actively practicing sin as leaders in the church (for example people that are leading an actively homosexual lifestyle, or adulterous lifestyle) and yet are still chosen by their congregation to be a leader of the people.