1 Corinthians 2:4-5:

And my speech and my preaching was not with enticing words of man’s wisdom, but in demonstration of the Spirit and of power:
That your faith should not stand in the wisdom of men, but in the power of God.

Notes on 1 Corinthians 2:4-5:

You don’t need to be a skilled orator to preach the gospel.  In fact, people complained about Paul’s speech and physical presence being contemptible.  People didn’t come to see him because he was good-looking or because he had such an eloquent way with words.  Some of his letters have some run-on sentences that would curl your English teacher’s hair.  People came to hear Paul because of the signs and wonders that followed his preaching of the gospel.

This doesn’t mean that you should make no effort to be a good public speaker.  There is no advantage in being unprepared, using bad grammar, and constantly saying, “You know, well, uh...” between every sentence.  However, you cannot trust in your fine words to convince men that Jesus is the Christ.  It is your job to proclaim it and the Holy Spirit’s job to do the convincing.

Paul didn’t say that he spoke poorly, though he admitted that others thought so.  He just said that he didn’t try to use man’s wisdom to make his arguments when he preached.  Instead, he preached Christ boldly and let the Lord convince men through signs and wonders.

Paul said much the same thing elsewhere:

1 Corinthians 1:17:
For Christ sent me not to baptize, but to preach the gospel: not with wisdom of words, lest the cross of Christ should be made of none effect.

When you hear preaching of the gospel, you should be more impressed with demonstrations than you are with the cleverness of the speaker.

See also:

Healing and Evangelism
Trust the Word to Work
Don’t Preach a D- Gospel