"The whole counsel of God concerning all things necessary for his own glory, man's salvation, faith and life, is either expressly set down in Scripture, or by good and necessary consequence may be deduced from Scripture: unto which nothing at any time is to be added, whether by new revelations of the Spirit, or traditions of men." (WCF 1.6)This idea can be found in Scripture, such as in 2 Timothy 3:15-17 where Paul tells Timothy,
"from childhood you have been acquainted with the sacred writings, which are able to make you wise for salvation through faith in Christ Jesus. All Scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness, that the man of God may be complete, equipped for every good work."It is true that God gave revelation which is not contained in Scripture. Not everything that the prophets prophesied or that Jesus spoke or that the apostles preached was written down. Yet, whatever is necessary for us to believe or obey was written down in Scripture and preserved for the ages to come. Because Scripture can give sufficient knowledge for salvation and can make the man of God complete, equipped for every good work (2 Tim. 3:15-17), then the one who knows Scripture does not need to fear that he is missing out on an additional doctrine or duty which is outside of Scripture and in oral tradition alone.
The tradition of the church is important, since Christ appointed pastors and teachers for the benefit of his church. There are some things in the Scriptures that are hard to understand, "which the ignorant and unstable twist to their own destruction" (2 Peter 3:16). Yet, these pastors and teachers are bound to teach what Scripture says, to give a faithful interpretation of what is there. Not only should they not contradict Scripture, but they should also not add more doctrines or duties to what is in Scripture, for Scripture contains all that we need to believe and obey.
As was evident in the Old Testament church, the oral tradition of the church can error. The leaders of the Jewish church had added to, contradicted, and wrongly interpreted the demands of Scripture. It was Scripture that was infallible and which Jesus and the apostles used to correct these errors (Matt. 15:1-9, Mark 7:1-23, John 7:21-23, Matt. 22:29-31). And so while some lingering oral tradition from the apostles may have given the early fathers help in faithfully interpreting the writings of the apostles, this oral tradition is not infallible and ought not to be relied upon to go beyond what is written in Scripture.