"And many of the Samaritans of that city believed in Him [Christ] because of the word of the woman who testified, 'He told me all that I ever did.' "
The testimony of believers--even brand new believers--(as above), is often profoundly used by the Holy Spirit in the conversion of other souls. This woman, whose reputation in town had been notoriously bad, now becomes the principal spokesman for the Messiah--and her neighbors believe along with her.
Never say to yourself, "I can't be effective for Christ--after all, my life has been a mess." Everyone's life is a mess. The key is not in our failed past, but in our perfect Savior. Don't hesitate to speak of Him.
[Puritan quote of the day: "Since Christ is thus comfortably set out to us, let us not believe Satan's representations of Him." --Richard Sibbes, in "The Bruised Reed"]