To add to the work of Christ is to say that what Jesus did on the cross isn't enough. This is an insult to God. The truth is important not because it is simply true. It is important because truth is what defines who and what we believe in. Only one is right.
Faith is vital.
But faith placed in something false is the same as having no faith at all. Faith is only as good as the object in which it is placed. That is why it is important to have the true Jesus. The one of the Bible, not the one of Mormonism, nor the one of the Jehovah's Witnesses. Eternity is a long time to be wrong.