From Gene Edward Veith's article on the Gospel Coalition Website:
"So are some occupations off-limits for Christians? No doubt, but it is not always clear what they are. Vocations are unique---that is to say, God calls and equips individuals in distinct and highly particular ways---so they may resist hard and fast and universally applicable rules and moralistic dictates. Since vocation is about God's work as well as human work, it has to do not just with the law but with the gospel; since vocation is where the Christian life is to be led, it will be an expression of Christian freedom."