It is a series of rhetorical questions. In effect, Paul is saying "In order to call on him, they need to believe. In order to to believe in him, they need to have heard of him, and in order to hear of him, they need to have someone preach to them, and in order for someone to preach to them, you need to send that someone." Each rhetorical question makes an inductive point - the first being that in order to call, they need to believe, the second that in order to believe, they need to have heard. But as the questions are rhetorical, he is assuming that these inductive points are already known. The putting them together in a logical chain - what the Romans apparently do not already know - is deductive.

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T W Hopper
Member, Presbyterian Reformed Church
Currently between churches since PRC closed here - attending Crossroads Christian Church.
Canberra, Australia.