From the time of the apostles until the present, the true church has always believed that the Bible is complete. God has given his revelation, and now Scripture is finished. God has spoken. What He gave is complete, efficacious, sufficient, inerrant, infallible, and authoritative. Attempts to add to the Bible, and claims of further revelation from God have always been characteristic of heretics and cultists, not the true people of God.
Although charismatics deny that they are trying to add to Scripture, their views on prophetic utterance, gifts of prophecy, and revelation really do just that. As they add--however unwittingly--to God's final revelation, they undermine the uniqueness and authority of the Bible. New revelation, dreams, and visions are considered as binding on the believers conscience as the book of Romans or the gospel of John.
Some charismatics would say that people misunderstand what they mean by prophetic utterance and new revelation. They would say that no effort is being made to change Scripture or even equal it. What is happening, they assume, is the clarifying of Scripture as it is applied or directed to a contemporary setting, such as the prophecy of Agabus in Acts 11:28.
The line between clarifying Scripture and adding to it is indeed a thin one. But Scripture is not clarified by listening to someone who thinks he has the gift of prophecy. Scripture is clarified as it is carefully and diligently studied. There are no shortcuts to interpreting God's word accurately (cf. Acts 17:11; 2 Tim. 2:15).
Christians must not play fast and loose with the issues of inspiration and revelation. An accurate understanding of those doctrines is essential for distinguishing between the voice of God and the human voice. Men who professed to speak for God but spoke their own opinions were to be executed under the Old Testament law (Deut. 13:1-5). New Testament believers are also urged to test the spirits and judge all supposed prophecies, shunning false prophets and heretics (1 John 4:1; 1 Cor. 14:29).
The Holy Spirit is working mightily in the church today, but not in the way most charismatics think. The Holy Spirit's role is to empower us as we preach, teach, write, talk, witness, think, serve, and live. He does lead us into God's truth and direct us into God's will for our lives. But He does it through God's Word, never apart from it. To refer to the Holy Spirit's leading and empowering ministry as inspiration or revelation is a mistake. To use phrases such as "God spoke to me," or "This wasn't my idea; the Lord gave it to me," or "These aren't my words, but a message I received from the Lord" confuses the issue of the Spirit's direction in believers' lives today.
Inviting that kind of confusion plays into the hands of the error that denies the uniqueness and absolute authority of Scripture. The terms and concepts of Ephesians 5:18-19 and 2 Peter 1:21 are not to be mixed. Being filled with the Spirit and speaking to one another in psalms and hymns is not the same as being moved by the Holy Spirit to write inspired Scripture.