The doctrine of the priesthood of believers (plural) does not mean that every Christian is a priest and, therefore, has the right to believe anything he desires. Rather, he is a priest in a covenant community of believers. The priesthood of believers is not the same thing as "soul competency." Soul competency declares that every human being has a knowledge of God (Romans 1:18-21) as is personally responsible to God and without excuse. The priesthood of believers requires every believer, as a part of a covenant community, to seek to guard his congregation from departing from the truth once delivered to the saints (Jude 3). This was the belief of the Reformers. Timothy George writes, "For them it was never a question of a lonely, isolated seeker of truth, but rather a band of faithful believers united in a common confession as a local, visible congregatio sanctorum (Founder's Journal). This is where a confession of faith becomes an essential part of a local church.
A historic confession of faith, like the London Baptist Confession of 1689, protects against the errors and heresies that can destroy a local church. What does the church believe about the essential doctrines of the Trinity or justification by faith or the person and work of Christ? Are the revelatory gifts still given to the church? It isn't sufficient to simply say, "My creed is the Bible." Every group claiming to be Christian claims the Bible as their authority including the Mormons and Jehovah's Witnesses. The issue is, what does the Bible teach? Our confession of faith declares what we believe the Scriptures teach.
As Baptists, we have never been creedalistic in the sense of placing a man-made statement above Scripture. We would surely never suppose any confession of faith to be infallible, but a historic confession provides the church with a statement of what they believe the Scriptures to teach. Spurgeon wrote regarding the 1689 Confession, "We are happy to join with centuries of believers in confirming this confession of faith. We do not take it as a substitute for Scripture, and we indeed must evaluate this and every other document by the light of Scripture. But it is a wise, organized, and useful statement of what the Bible teaches; a guide as we search the Scriptures and examine our own teachings and practices; and a way of affirming our unity with the many Christians who have treasured these doctrines."
To be a member of our church, one does not have to fully subscribe to every part of our confession. We recognize the duty of every Christian to interpret the Scriptures according to their conscience. But we explain clearly to every prospective member what our church believes about the essential doctrines of Scripture. Our confession protects our church from the division and schisms that may arise in the absence of a doctrinal standard. And while a person doesn't have to fully subscribe to our confession in order to be a member, we do demand full-subscription in order to be an officer in our church. Every pastor/teacher must profess full-subscription to our confession. This protects the doctrinal integrity of our church and gives the congregation confidence in what is being taught.
The issue really isn't whether an individual Christian has the right to interpret the Scriptures according to the dictates of his conscience, but whether the local congregation has the right and duty to clearly state the doctrinal parameters of the church. It is essential that these doctrinal boundaries be clearly stated. This is accomplished by way of a confession of faith.
By way of a final point, there is great value in adopting a historic confession that has been tried and tested by the Christian community for many years. There is no need to "reinvent" the wheel. Again, Spurgeon wrote of the Second London Baptist Confession, "We are happy to join with centuries of believers in affirming this confession of faith." Trinity Baptist Church heartily agrees, and has also confirmed the 1689 Confession as a faithful expression of the doctrine taught in the Scriptures.