I heard some preachers say that, if you are ill, like having cancer, you should pray to God for healing, instead of see doctor. Is this teaching ok?
No it isn't good teaching. If anyone believes in God then they should know that God made doctors to go to so that is where anyone should go to get help. That is if you believe in God.
Prayer is no more ineffective than chemotherapy, and considerably cheaper. American medicine is, unfortunately, far removed from science and quite close to corporate interests. At present, it isn't much better than prayer.
