Why are some denominations so against the idea of personal revelation as a source of Truth?

First let's clarify what the contents of Scripture are.

We should put away from our mind that the material is unique, non repetitive. Because it is meant to teach, it adopts the standard practice of teaching by repetition, and by incremental enlargement of the repeated teaching, what the educationists call the educational spiral. Children learn addition in junior class, and multiplication (which is a sophisticated form of addition) in the next class. IOW, building on a previous teaching. Which theologians call progressive revelation.

2 Timothy 3:16 New American Standard Bible (©1995) All Scripture is inspired by God and profitable for teaching, for reproof, for correction, for training in righteousness;

In the sense that Scripture is written by God, when men spoke from prompting by the Holy Spirit, no more new revelation is being made. All the teachings are in place, both in in terms of uniqueness, and in richness.

2 Peter 1:20 NIV Above all, you must understand that no prophecy of Scripture came about by the prophet’s own interpretation. 21For prophecy never had its origin in the will of man, but men spoke from God as they were carried along by the Holy Spirit.

However, prophecy today carries the associated meaning of teaching, which is also what it sometimes meant in Scripture:

1 Corinthians 13:8 NIV Love never fails. But where there are prophecies, they will cease; where there are tongues, they will be stilled; where there is knowledge, it will pass away. 9For we know in part and we prophesy in part, 10but when perfection comes, the imperfect disappears. 11When I was a child, I talked like a child, I thought like a child, I reasoned like a child. When I became a man, I put childish ways behind me. 12Now we see but a poor reflection as in a mirror; then we shall see face to face. Now I know in part; then I shall know fully, even as I am fully known. 13And now these three remain: faith, hope and love. But the greatest of these is love.

So when the church assembles, prophecy takes place, not as fresh material being added to the corpus, but as CHOOSING of teaching relevant to the circumstances of the church and its interpretation of that chosen text or topic.

The vineyard has been handed to its new tenants:

Matthew 21:40“Therefore, when the owner of the vineyard comes, what will he do to those tenants?” 41“He will bring those wretches to a wretched end,” they replied, “and he will rent the vineyard to other tenants, who will give him his share of the crop at harvest time.”

If the term "personal revelation" is used in the sense of making correct sense of the material of Scripture, then it has not ended. That is because you can transmit the form but not the content. Content can and has been lost with passage of time, as can be seen with churches which hold to interpretations which are distorted doctrine, considered distorted because they have clearly deviated from Scripture, and have been slow to correct those distortions.

In this regard, new forms of churches have begun to rise, some labeled post modern, others emergent and still others, organic, with varying success in tackling the problem of being faithful to the interpretation of Scripture.

The views of the organic church is that interpretations are lost and need to be constantly corrected, just as a ship's course needs to be constantly checked and brought back on track, with reference to a map:

1 Corinthians 14:26 What then shall we say, brothers? When you come together, everyone has a hymn, or a word of instruction, a revelation, a tongue or an interpretation. All of these must be done for the strengthening of the church. 27If anyone speaks in a tongue, two—or at the most three—should speak, one at a time, and someone must interpret. 28If there is no interpreter, the speaker should keep quiet in the church and speak to himself and God.

On a loosely related question about the nature of the doctrine of Sola Scriptura and external sources of Truth, Jas 3.1 gave an excellent answer.

Since Scripture adequately equips you to correct false doctrine, that implies it is the standard of truth.

So the point is this: If a person teaches something contrary to Scripture, they are wrong.

That is not to say that everything outside of scripture is wrong (or "not from God")... just everything contrary. Therefore we reject any teaching which opposes what we know (based on Scripture);

This is in line with, for example, the Southern Baptist statement of belief on the nature of Scripture:

The Scriptures

The Holy Bible was written by men divinely inspired and is God's revelation of Himself to man. It is a perfect treasure of divine instruction. It has God for its author, salvation for its end, and truth, without any mixture of error, for its matter. Therefore, all Scripture is totally true and trustworthy. It reveals the principles by which God judges us, and therefore is, and will remain to the end of the world the true center of Christian union, and the supreme standard by which all human conduct, creeds, and religious opinions should be tried. All Scripture is a testimony to Christ, who is Himself the focus of divine revelation.

To the groups that I am thinking of, the idea of continuing revelation isn't necessarily dangerous in and of itself, if that revelation is in accordance with the teachings of Scripture. If God doesn't change, any revelation that comes from Him will be in accordance with what He has already revealed to us.

The problem is, from a Fundamentalist perspective, that many Christians seem to believe that personal revelation is on an equal or higher level of authority as Scripture.

In the words of Spurgeon:

Lots of well-meaning Christians, however, seem to operate with the misconception that biblical revelation is somehow exempt from the law of contradiction. They suggest that God's truth can contravene logic if God is so pleased. They often point to the doctrine of the Trinity or pit divine sovereignty against human responsibility as evidence that revealed truth is sometimes contradictory.

But Titus 1:2 tells us that "God . . . cannot lie." Therefore even God's Word must be in harmony with the law of contradiction. One clear, unresolvable contradiction would be enough to destroy the trustworthiness of the whole. That's why the enemies of truth are so eager to try to prove that God's Word contradicts itself.

A more contemporary preacher, John MacAurthur also addresses this, nothing the disastrous, destructive results of misinterpreted "personal revelation from God".

Put simply, from a Fundamentalist perspective, once a person believes that personal interpretation, personal revelation, or personal anything is equal to, or greater than God's revealed Word, that person can believe in anything they want.

Example: http://www.karenkober.net/karen.html

Karen is one of twelve disciples chosen by God to follow His Son, Yeshua. Called into the desert in May of 2003, she committed to God for His Son. Upon the commitment, God gave her the Nine Gifts of Discernment. John the Baptist told her, "As I have opened the path for Him to come, you are to open the path for Him to come again.". She is to create a Sacred Garden for Yeshua and His Mother to welcome the masses. She is to spread the word around the world of His Second Coming. Yeshua will be seen to her right and the Blessed Mother to her left. Those with spiritual eyes have already seen Mother walk alongside of Karen. About 2000 years ago, Maria, Mariamene and Salome/Miriam walked with Yeshua. Karen was the "Third Mary" at the cross.

I know this woman personally, although I cave been out of contact with her for several years, I can attest that she believes this, as well as all the other stuff on her website to the very core of her being. She didn't always believe these things. She believes them now because she rejects Scripture as "man-made", and also because "God revealed these things" to her personally. No amount of logic, reason, or Scripture can make a dent in her unshakable faith in the things that have sprung from her own imagination.

It's the perfect example of what is spoken of in Romans 1

Romans 1: KJV

21 Because that, when they knew God, they glorified him not as God, neither were thankful; but became vain in their imaginations, and their foolish heart was darkened.

22 Professing themselves to be wise, they became fools,

23 And changed the glory of the uncorruptible God into an image made like to corruptible man, and to birds, and fourfooted beasts, and creeping things.

From a Fundamentalist standpoint, this is dangerous to her because the embrace of such anti-Christian beliefs means that she will never put her faith in Christ alone. Her faith is in the "God" that she has created in her own mind. On a personal level, this breaks my heart. And she is actively leading others to follow her, which is dangerous to them as well.

Her beliefs sprung from a belief in God. She believed in Jesus. (Still does, just not the Jesus revealed in Scripture, but rather one she's invented.) But she believes that her own personal understanding, and that the revelations that God gave her are of higher trustworthiness than a Bible that was "made up".

This is where, from a Fundamentalist point of view, personal revelation is distrusted, and seen as dangerous. Not because we believe God can't and doesn't use personal revelation. In point of fact, we do believe that God speaks to us through personal revelation:

• We believe He speaks to us in prayer (perhaps not audibly, but by giving us comfort when we are in His will, and allowing us to sense when we are not - be it unease, uncertainty, etc.)
• We believe that He reveals His will to us in many ways.

We just believe that God will not contradict Himself, and that any personal revelation absolutely must be measured using the plumb-line of Scripture. The danger comes from refusing to check the personal revelation against that plumb-line, and falling into all sorts of false doctrines.

Failure to test personal revelation is a failure to apply 1 John: 4:1 (KJV)

Beloved, believe not every spirit, but try the spirits whether they are of God: because many false prophets are gone out into the world.

This leaves us wide-open to Satan's lies. He can be pretty convincing, and Scripture warns us that he can disguise himself as an angel of light in order to deveive us in 2 Corinthians 11:14

And no marvel; for Satan himself is transformed into an angel of light.