In order to adhere to the 9th commandment, one is required to be authentic about his or her beliefs. People who lie about their beliefs give false testimony about themselves. They use deceit in order to appear differently to other people.
In the world we live in today, it is quite common for Christian people to deny their beliefs. They do this because morality has been corrupted over generations, and the righteous have become a minority. These people fear that if they expose their true beliefs, the majority will think of them as odd, overly conservative, uptight, judgmental, or naïve.
For example, a girl who is a devout Christian witnesses her peers speaking cruelly about a boy behind his back. The girl knows that what they are doing is wrong, however, she does not want to confront her peers about this in fear of being rejected. Instead, she stays quiet, hoping nobody will notice her silence. When someone finally asks her what she thinks of this boy, she agrees with the majority. She, too, hesitantly begins saying negative things about him, even though she does not mean it.
The girl values acceptance from her peers so much so that she went against her own principles in order to fit in. She hid her beliefs against backstabbing and bullying, in fear of being backstabbed and bullied herself. This is a violation of the 9th commandment because instead of upholding the truth about her beliefs, she denied them and distorted her image for the sake of social approval.