Before we can go any further I believe we must change one word in the question to reach the central question. "What" should be changed to "Who". It is not a "what" that qualifies men to speak for God but a "Who". If we spend our time on the periphery "what", then I believe we will miss the biblical answer. First, we must discover "Who" qualifies men to speak for God and His Word, then we can begin asking questions about the "what".
As I set out to answer this question what I found was one prominent thing; God alone qualifies men to speak for Him and His Word. Who qualified Moses to speak for God? (Exodus 3-4) Who qualified Samuel to speak God's Word? (1 Samuel 3) How does Nathan have the authority to rebuke King David? Who qualifies Him to speak? (2 Samuel 12:1) Who qualifies such prophets as Isaiah, Jeremiah, Ezekiel? (Jeremiah 1, Isaiah 6, Ezekiel 2) Who qualifies every other prophet to speak? The answer to all of the above questions is God Himself.
This is not only found in the Old Testament but also the New. We see this clearly in 2 Corinthians 3. Paul is asking the Corinthians, "do we need letters of recommendation?" Then it is as if he says, "you know our ministry, you have experienced what God has done, it speaks for itself, you know that we are qualified." But just to make it clear he lets them know where their authority comes from. He answers our question: Paul, "who has qualified you to speak for God and His Word?" His answer: "Such is the confidence that we have through Christ toward God. Not that we are sufficient in ourselves to claim anything as coming from us, but our sufficiency is from God, who has made us competent to be ministers of a new covenant, not of the letter but of the Spirit. For the letter kills, but the Spirit gives life."
Much of the dialogue in John 5-10 is in response to this very question. What makes Jesus think that he has the right to speak for God and His Word? What qualifies you to say and do such things, Jesus? Jesus continues to point to the Father as His source of authority: "I have come in my Father's name...My teaching is not mine, but his who sent me...I have not come of my own accord..." Now certainly Jesus is God so his situation and authority is different than ours. We do not have inherent authority, Jesus did. But, it is significant that He still points to the Father as the one that "qualifies Him" to speak for God and His Word.
This does not fully answer the question asked though. Very few people would say that our authority does not come from God. The question before us then is this: "How do we know whether or not God has qualified someone to speak for Him and His Word"?
Do we look at the qualifications for an elder and say, "only men with these qualities can speak for God"? Do we consult the local church, and say, "those set apart by the local church, they alone can speak for God"? Do we test the fruits of that which is said, and by the fruit discern whether it is from God? Or is it possible that it is a combination of these?
I can understand the concerned motivation behind this question. American Christianity is a breeding grounds for "thus-saith-the Lord" heresy. Just a few days ago I linked to Pat Robertson's New Year's predictions. Is Pat Robertson qualified to make such a statement? He is an ordained minister. At some point a local church has set him apart as a messenger of God. He has degrees. He has qualifications. But do we see fruit? Do his predictions come true?
Can we say that Joel Osteen preaches the word of God? How do we know John MacArthur preaches God's Word? What about John Piper? What about myself on a Wednesday night? How do we know whether or not God has sovereignly chosen to speak through someone or not? That is a profound question. It may just take a 208 page book to even begin discovering how to be more discerning in these matters. In brief I want to offer two suggestions on how we can discern whether the message is from God or not.
The first comes from 1 John. "Beloved, do not believe every spirit, but test the spirits to see whether they are from God, for many false prophets have gone out into the world. By this you know the Spirit of God: every spirit that confesses that Jesus Christ has come in the flesh is from God, and every spirit that does not confess Jesus is not from God. this is the spirit of the antichrist, which you heard was coming and now is in the world already. Little children, you are from God and have overcome them, for he who is in you is greater than he who is in the world. They are from the world; therefore they speak from the world, and the world listens to them. We are from God. Whoever knows God listens to us; whoever is not from God does not listen to us. By this we know the Spirit of truth and the spirit of error"
It would take an entire sermon (and much more) to fully exposit this text. For now I want to show three things that help us to see whether it is from God or not. How do you know if God has sovereignly decided to speak through someone?
1) If they have an accurate Christology as clearly revealed in Scripture then perhaps it is from God.
2) If their teaching sounds like the world, then it is not from God. (This is where the Osteen's and prosperity teachers fall off). If their teaching matches up to what is revealed in Scripture then it should be received.
3) What do trusted believers say concerning the matter? Since the Holy Spirit dwells in believers and he confirms truth, what is the consensus of godly men. Since this one is the most subjective it is the most unreliable, but still should be strongly considered. Ultimately our trust in discerning these matters should be in the Lord.
The second suggestion comes from Jeremiah Burroughs. A recent reprint of some of Burroughs' sermons entitled Gospel Fear might give us an answer. After discussing our need to tremble at God's Word (and Burroughs includes in that the preaching of that Word) he pauses to address an objection. "...we will fear the Word of God; but is everything that a minister speaks the Word of God? If we were sure that it was God's Word, then we would yield unto it. But we know that one man is of one opinion and another is of another opinion." In other words, how do we know whether it is God's Word? Burroughs answer is beautiful and very fitting for our current discussion:
When anything comes in God's name, do not slightly cast it off, but try it and examine it...So do not lightly cast off that which comes in the name of God, but yield so far unto it as to examine it. Search the Scriptures; test whether it is according to the divine rule or not. I know a gentleman who, when he came home once from a sermon, said, 'Well, if it is true what this minister says, we are in an ill case'. Now woe to that man whose chief comfort lies upon this false ground, that he hopes that which he hears out of the Word is not the Word. Oh, that man is in a miserable case who has no other ground for his comfort" Gospel Fear, p23Burroughs then offers two suggestions for those that are poor and ignorant. First of all be willing to yield to what you know. Secondly, get alone and cry to God, and ask the Lord that he would make His Word known to you.
In sum, what (or rather who) qualifies someone to speak for God and His Word? It is God alone. But, how can we know whether or not God has sovereignly chosen to speak through someone? We should not slightly cast it off if it comes in God's name but we should test it. There is something to be said for the local church standing behind a man. But they can be wrong (either positively or negatively). There is something to be said for elders being the primary teachers. But certainly God can speak through laity (He even chose to speak through a donkey). But ultimately, we must rely upon the Lord and His Spirit to help us discern whether or not it is God's Word. God is sovereign and the Spirit inside us will help us discern His truth.
Perhaps this adds little to the conversation, but may God be glorified.