If you can help Christians answer this question for themselves, for their own lives and their own ministry involvement, you help them take a big step towards maturity as they face fear with the god who provides and promises to look after them, even while their worst fears come true before their eyes.
I reckon this is the big question to get people to answer for themselves as they approach thinking about doing MTS and a life of full-time ministry. If they can identify their deepest fears about giving their life to the work of the gospel, it might not make the decision easier, but it will afford them the chance to grow in their love and dependence on God.
Don’t ask it straight off the bat… You gotta work to get that deep.