There's nothing you can do to MAKE them believe in God; And nothing you can do to get them to not believe in, and partake in, the activities they are. You have reached out and preached the Gospel to them, that's about all you can do. Something you'll realize is that you're never going to be able to FORCE someone to believe. Simply carry on with your on FAITH and continue to live for the Lord. You can't allow them to cheat you out of your eternal life. You have to work on your OWN faith if its that's shaky. And I don't mean that in an insulting way either.
I will pray for you, and I will pray for them, too. And I strongly suggest you continue to seek God, and pray for them.