Thanks for commenting. As to your first question James 2:19 says the demons believe there is but one god. James isn't discussing salvation. The demons shudder because the know God in an intimate. They dwelled with Him until their rebellion.
Regarding the rest, I am speaking to the question What MAKES one a Christian. It isn't works. It's faith and nothing more. Specifically, faith in Jesus Christ. That's not to say we we shouldn't add works to our faith. That's why 2 Peter 1:5-9 says:
For this very reason, make every effort to add to your faith virtue; and to virtue, knowledge; and to knowledge, self-control; and to self-control, perseverance; and to perseverance, godliness; and to godliness, brotherly kindness; and to brotherly kindness, love. For if you possess these qualities and continue to grow in them, they will keep you from being ineffective and unproductive in your knowledge of our Lord Jesus Christ. But whoever lacks these traits is nearsighted to the point of blindness, having forgotten that he has been cleansed from his past sins.
Notice that he doesn't say lacking these qualities loses your salvation. He says it makes you ineffectual. It causes people to forget that they've BEEN CLEANSED from past sins. If you've been cleansed, you're a Christian.
James encourages Christians to move beyond mere belief and to take action, proving one's faith is effective.