Faith is the sole instrument by which a person may receive the blessing of Christ’s righteousness imputed to him. Now when we say “sole instrument” we do not mean that no other instruments are involved, but that no other instruments are involved in the way that faith is. Faith is the capstone of all instruments, and it is the only capstone. This is what that means.
Faith comes by hearing, and hearing by the Word of God. How shall they hear without a preacher? So, is preaching therefore an instrument of justification? Yes, a secondary instrument. This means that there is not a one-to-one correspondence between hearing a preached message and justification. Baptism in water is a secondary instrument, meaning the same thing. Lots of baptized people die and go to Hell. Lots of preached at people die and are lost forever.
The only instrumental cause of justification that leaves no remainder is faith. Every person who exercises the God-given blessing of faith is justified, and will be glorified at the last day, head for head. There is a one-to-one correspondence between those who trust in Jesus as a result of the effectual call and those who are saved. No one is saved who did not exercise that faith at some level in some way, and everyone who exercises that faith in that God-given fashion will be preserved to the last. This is what we mean when we confess sola fide. Faith alone is the instrument of justification.
There is a vast hierarchy of instruments below that — many bricks come below the capstone. But there is only one capstone. But none of those lesser bricks, lesser instruments, result in salvation necessarily. True evangelical faith does.
Preachers, books, seminaries that train preachers, baptisms, tract publishers, church services, evangelistic beach trips . . . none of them are 100% effective. But when the Spirit creates faith — that is 100% effective.
Faith is the active agent. Faith is the catalyst that makes any of these other things worthwhile. The word preached does nothing unless mixed with faith (Heb. 4:2). If you baptize someone who is faithless, you have a faithless wet person. That faithless wet person is now covenantally obligated to not be faithless, but his obligations to not be faithless are not synonymous with actually being faithful. And you cannot be faithful apart from, you guessed it, faith.
For those who are following this, and who want to know what the FV fuss is about then, consider it a function of evangelical suspicions about one secondary instrument (baptism), suspicisions they do not have about another secondary instrument (preaching). I am equally suspicious of both, if faith is absent, and equally willing to whoop in favor of both, if faith is present. The problem of the TRs at this point is not that they are too suspicious of secondary means, but that they are not suspicious enough.
The Bible contains warnings, true enough, about sacramental obedience that has been detached from heart obedience — circumcise your hearts, the prophets tell us. But the Bible also gives us warnings about great preaching conferences. The one who hears the word proclaimed but does not do it deceives himself (Jas. 1:22). And the more that the conferences preachers thunder like Whitefield, or Spurgeon, or Bunyan, the worse the deception gets. Doesn’t it?