For the fruit of the light consists in all goodness, righteousness and truth.
Truth is a fairly basic concept for Christians and non-Christians alike. We were raised being told to tell the truth. It is also at the very core of Christianity that we are not to bear false witness, or in other words, to lie.
But the truth extends beyond just not lying. Christians are witnesses of Jesus Christ. We are a testimony to the power of God through the difference in our lives. This must be told to others.
The name Christian literally means little Christ. When we take the name Christian, we claim Christlikeness as well. To deny that we are Christians either verbally or by the way we act is to avoid the truth.
We do not have to be obnoxious about our faith but we should not hide who we are either. Many Christians are Sunday morning Christians only. They have no problem showing up to church but do not talk about it throughout the week.
Being truthful includes being open about your Christianity. You should do nothing to hide it but instead be proud of your identity in Christ.