God is by far the greatest preacher in the universe. God doesn't even have to use words to communicate to his image bearers how great and awesome he is. Psalm 19:1-4:
The heavens are telling the glory of God;
and the firmament proclaims his handiwork.
Day to day pours forth speech,
and night to night declares knowledge.
There is no speech, nor are there words;
their voice is not heard;
yet their voice goes out through all the earth,
and their words to the end of the world.
Nature--hence the term "natural revelation"--is mute testimony to God's "invisible nature; namely, his eternal power and deity . . . (Romans 1:20, excerpts, RSV).
Then there is special revelation and the multitudinous ways in which God has chosen to reveal Himself to his image bearers:
- His written word
- fire, smoke, wind, and other "natural" phenomena which He harnesses for His own purposes
- angelic messengers
- miracles, signs, and wonders
- a voice from above, as at Jesus's baptism and transfiguration
- the controlling of circumstances, as when Paul, for example, was prevented by the Holy Spirit to venture into Asia (Acts 16:6)
- the Eternal Word, Jesus, who is the greatest of God's special revelations
There are many more examples of special revelation, but my short list covers many of the salient ones.
One factor tying the many examples of special revelation together is their unpredictability (from the image bearers' perspective), and that unpredictability flows from God's eternal counsels and sovereign will.
From Paul's testimony of his conversion, we see a number of special revelations at work. Paul was
- set apart by God from his mother's womb
- called through God's grace
- chosen to have God's Son revealed in him
- commissioned by God to preach Jesus among the Gentiles
- told by God to go up to Jerusalem to inform the apostles of his commission by God to preach to the Gentiles (see Galatians 1:15-2:10)
None of the above would have happened, however, were it not for Jesus's revelation of himself to Paul on that fateful day when Paul, then Saul, was wending his way to Damascus to arrest more followers of the Way, and Jesus revealed Himself to Saul, first as a light from heaven and then as an audible voice (Acts 9:3-6).
According to Paul's testimony before Governor Festus and King Agrippa, God gave Paul, the master of rhetorical questions, a rhetorical question of His own:
Saul, Saul, why are you persecuting Me? It is hard for you to kick against the goads (Acts 26:14 RSV).
And so finally, we come to an answer to your question, which is simply an iteration of my first paragraph: God is the greatest preacher, and has both the ability and the right to bypass human instrumentality (e.g., the words of preachers, teachers, apostles, and prophets) and break through with His will and Word by way of special revelation to anyone he chooses, at any time, any place, and in any circumstance.
As important as preachers and servants of the Word are, their importance pales in comparison to the power, primacy, and permanence of the living and abiding Word of God.