Take the helmet of salvation and the sword of the Spirit, which is the word of God. (NIV)
And again in Hebrews 4:12.
For the word of God is living and active. Sharper than any double-edged sword, it penetrates even to dividing soul and spirit, joints and marrow; it judges the thoughts and attitudes of the heart. (NIV)
In modern parlance, we would call this conviction of sin, one of the states of unregenerate man that precedes repentance. The other great result of preaching the word of God is faith. Faith is caused by the preaching of Christ.
Consequently, faith comes from hearing the message, and the message is heard through the word of Christ. Romans 10:17 (NIV)
A command to use the sword of the Spirit is in 2 Timothy 4:2.
PREACH THE WORD; be prepared in season and out of season; correct, rebuke and encourage--with great patience and careful instruction. (NIV)
F.W.H. Meyer in his narrative poem St. Paul said it this way:
Then I preached Christ, and when she heard the story,
Oh? is such triumph possible to men?
Hardly, my King, had I beheld Thy glory,
Hardly had known Thine excellence till then.
Then with a rush the intolerance craving
Shivers throughout me like a trumpet call,
Oh! to save these; to perish for their saving;
Die for their life; be offered for them all.
(Taken from Weapons & Tactics in Evangelism, part 7)