Many protestants believe that all the need to do is have faith in God, and they will be saved.
This is a path to damnation and hell. Consider that Satan has faith in God, absolute faith. Yet, Satan is damned to hell. Why? He was wicked.
Christ said "by their fruits you shall know them". If you bear bad fruit, you are not saved. It doesn't matter if you have faith - Satan has faith.
If you have faith in the LORD, but reject and persecute his Church, deny his teachings, and defy his commands, you will land in hell and saying "Lord, Lord!" will not avail you.