A Calling is the requirement

Any ministry of leadership in Christ's Church, ordained or not, thus presupposes at the deepest level a calling by the Spirit of God and of Jesus Christ, who breathes where he will and calls whom he will and whose willing instruments both senders and sent are permitted to be.—Why Priests?, page 86

...it must be said that charism in the strict sense, i.e., a calling from God in the Spirit of Jesus Christ, stands by itself and does not flow from the institution. It is a free calling to a free ministry in the Church, which the Church leadership can suppress or even worse extinguish only at its own expense. A thoroughgoing direct or indirect “bureaucratizing” of a charism contradicts the New Testament. As the New Testament shows, a charism has no need at all of prior legitimation by a Church institution. On the contrary, there are in fact institutions and representations of institutions who have nothing charismatic about them: for instance, ordained Church functionaries who carry out their ministry mechanically and show no sign of a genuine calling of of the Spirit of Christ.—Why Priests?, page 87

<idle musing>
The sad part is that he is correct about the last part; too many are doing things they are not called to do—and not just in the "ordained" ministry. Have you ever been guilted into doing something "for the Lord"? If so, you are probably a victim of what he is talking about.
</idle musing>