One can obtain faith by keeping your focus on Jesus Christ and learn not on your own understanding, alknowledge him in all your ways and he will direct your patch.

You can obtain faith by believing in Jesus Christ as your Lord's and personal saviour.

Christ Jesus had give his desciples the authority to cash out demons. He said to them, if you believe in me you should do more than I have done  in the name of my father's who is in heaven.

The desciples could used the authority give to them by Jesus.