Short answer, no, you can't have truth that is only known through spiritual means.
You can have truth known through spiritual means, but that "only" in the question makes all the difference.
Human understanding is based on experience, observation, sometimes reason, emotional frame of mind, and a whole host of other factors. What we believe to be truth is shaded by what we observe around us and how we've been conditioned to interpret what we observe.
So, from a purely logical perspective, the answer is "No. There can not be a truth that we know only via spiritual means". Anything we " know" has already been filtered out by our thought process, using logic applied to past experience and observation.
The fact that we've evaluated the thought to determine if it's true already has nullified the "only" in your question.
That's from a purely logical perspective. Moving on to a "Christian" perspective:
The big question is "how do you determine what is truth? What is your plumb-line - your test for determining if something is true?" Is it something tangible? Is it reliable?
To better answer your question, I'm going to compare my viewpoint - one that holds the Bible as the ultimate authority on truth to the proposed viewpoint: the idea that spiritual discernment is the ultimate authority on truth.
From my view, Scripture is the plumb-line. Any doctrinal truth must be traced back to one of the following:
- A direct, clear statement from Scripture on a subject
- example: Theft - thou shalt not steal
- Logical reasoning that takes it's base supporting argument from Scriptural statements.
- example: racy movies based on what Scripture teaches about lust, and particularly the verse "I shall set no evil thing before my eyes". The Bible doesn't mention movies, but using the Bible as a plumb line, it's not hard to come to that conclusion.
Now on the other hand, if spiritual revelation is an equally valid "plumb line", then there is no such thing as truth. I can have a spiritual revelation that says that racy movies are actually holy because God created our bodies for enjoyment. I can claim to have divine revelation or the "leading of the Holy Spirit" that led me to understand that there's nothing wrong with theft, or that it's OK to lie.
In short, I can believe whatever I want.
Such a viewpoint would be patently un-Christian. Since this is a site about Christianity, I have to assume that you want a Christian perspective. Christianity, whatever flavor, is based on the God of the Bible.
There may be disagreements about certain teachings in the Bible, and varying opinions on the inerrancy, authority and reliability of the Bible, but if we take away the Bible's authority, we're left with nothing upon which to base our Christian claims of faith. W're left with simply "I believe" with no frame of reference, no plumb line, so we can believe whatever we want.
If we allow spiritual discernment or personal revelation to take co-equal status with Scripture, then we may as well decide to worship smurfs and sacrifice rodents to our "god" because we'll no longer be worshiping God as He revealed Himself, but instead we'd be worshiping gods that we made with our minds - idols.
This isn't to say that there is no Spiritual revelation, or that Spiritual revelation doesn't have its place. What I'm saying is that even true, honest spiritual revelation must meet certain tests, as laid out in Scripture.
The three key passages that come to mind are:
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.
2 Timothy 3:16 (KJV) All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for
instruction in righteousness:
John 4:24 (KJV) God is a Spirit: and they that worship him must worship him in spirit and in truth.
No doubt these false prophets either intentionally deceive, or else they honestly believe what they're teaching. We're told to test the spirits and see if what they are saying lines up with Scripture.
Therefore, even if there is "truth" that is spiritually known, we are commanded as Christians to test it to see if it meets the standard - does it agree with the rest of Scripture? Does it directly contradict Scripture?
And because of this command from Scripture, the Biblical answer to your question is a resounding "no". There cannot be truths that are only known through spiritual means. They must be known through spiritual means and via logical comparison to existing known doctrinal truth as revealed in Scripture.