The last words of Christ to His disciples were to go into all the world and preach the gospel. This is also refered to as "The Great Commission".
Here are some scriptures:
And he said unto them, Go ye into all the world, and preach the gospel to every creature. 16 He that believeth and is baptized shall be saved; but he that believeth not shall be damned. Mark 16:15-16
Who also hath made us able ministers of the new testament; not of the letter, but of the spirit: for the letter killeth, but the spirit giveth life. 2 Corinthians 3:6
Preach the word; be instant in season, out of season; reprove, rebuke, exhort with all longsuffering and doctrine. 2 Timothy 4:2
But sanctify the Lord God in your hearts: and be ready always to give an answer to every man that asketh you a reason of the hope that is in you with meekness and fear 1 Peter 3:15
And they overcame him by the blood of the Lamb, and by the word of their testimony Revelation 12:11