Proverbs 25:18 NKJV

[18] A man who bears false witness against his neighbor is like a club, a sword, and a sharp arrow.

In the book "Hedges" by Jerry Jenkins, he tells a story about a bright young evangelist who had his ministry (and his reputation) upended by a woman who accused him of inappropriate contact. Although the woman later admitted that she had come onto him and he rejected her (and her allegations had been false), the damage was already done. Her words were untrue, but the fallout was very real.

As followers of Jesus, we have an obligation to always seek truth. Taking it one step further, we are also required to bear with and speak truth no matter the circumstance. If we profess ourselves to be Christ followers and choose to speak lies regarding another person, we are bearing false witness against them and against the Gospel.