1Cor 9:14-18
"In the same way, the Lord commanded that those who proclaim the gospel should get their living by the gospel. But I have made no use of any of these rights... For if I preach the gospel, that gives me no ground for boasting. For necessity is laid upon me. Woe to me if I do not preach the gospel!"
Paul had the right to be supported by the Church (9:14; Lk 10:7), but he voluntarily renounced it so as not to create an obstacle to the Gospel. His missionary compulsion: 'woe to me if I do not preach' (ouai gar moi estin) — it is not an option, it is a necessity.