Perhaps you have heard a reformed pastor claim that when he stands behind the pulpit and preaches, you must listen because Christ is speaking through him. I have. I find it a bit of an odd claim because they imply there is something unique about their office and their function within the corporate gathering that grants them this authority. For example, in a brief article discussing the difference between preaching and teaching, Barry York says
Speaking for Christ versus speaking of him. Perhaps the most daunting aspect of preaching is that the minister is speaking on behalf of the Lord. Paul makes that clear when he says this of preaching:
How then will they call on him in whom they have not believed? And how are they to believe in him of whom they have never heard? And how are they to hear without someone preaching? And how are they to preach unless they are sent?” (Romans 10:14-15)
James Boice has pointed out that the word “of” in the statement “And how are they to believe in him of whom they have never heard?” is not there in the original. Rather, it should read “And how are they to believe in him whom they have never heard?” As men are sent out to preach, Christ through his Spirit is speaking through them. As Paul said elsewhere, “We also thank God constantly for this, that when you received the word of God, which you heard from us, you accepted it not as the word of men but as what it really is, the word of God, which is at work in you believers” (1 Thes. 2:13). Teaching can tell wonderful things about Christ, and every Sunday school class should do so. Yet only duly ordained ministers in preaching can make the authoritative claim that they represent the Lord.
This certainly does not follow from either Romans 10 or 1 Thes. 2. There is nothing about the office of elder that grants them an exclusive claim to be speaking for Christ.
And yet, it is true that Christ speaks authoritatively through preaching. Note Owen on Hebrews 1:5.
That it is lawful to draw consequences from Scripture assertions; and such consequences, rightly deduced, are infallibly true and “de fide.” Thus from the name given unto Christ, the apostle deduceth by just consequence his exaltation and pre-eminence above angels. Nothing will rightly follow from truth but what is so also, and that of the same nature with the truth from whence it is derived. So that whatever by just consequence is drawn from the Word of God, is itself also the Word of God, and truth infallible. And to deprive the church of this liberty in the interpretation of the Word, is to deprive it of the chiefest benefit intended by it. This is that on which the whole ordinance of preaching is founded; which makes that which is derived out of the Word to have the power, authority, and efficacy of the Word accompanying it. Thus, though it be the proper work and effect of the Word of God to quicken, regenerate, sanctify and purify the elect, — and the Word primarily and directly is only that which is written in the Scriptures, — yet we find all these effects produced in and by the preaching of the Word, when perhaps not one sentence of the Scripture is verbatim repeated. And the reason hereof is, because whatsoever is directly deduced and delivered according to the mind and appointment of God from the Word is the Word of God, and hath the power, authority, and efficacy of the Word accompanying it.
It is not the office that determines the authority, but whether or not the preacher makes correct deductions from Scripture. And this same authority is true anytime anyone makes a statement that is correctly deduced from Scripture, whether they are ordained or not, whether it is in the corporate gathering or not. Thus, contrary to York, teaching can speak for Christ just as much as preaching can. Note Augustine “Yes it is I who admonish, I who order, I who command, it is the bishop who teaches. But it is Christ who commands through me.” “The preacher explains the text; if he says what is true, it is Christ speaking.”