Here is a valid, and perhaps even sound, argument. Kierkegaard would worry that 4 begs the question.
- (Premise) If something is naturally impossible and it occurs, it occurs by miracle.
- (Premise) If a proposition is incredible, it is naturally impossible to believe it.
- (Premise) Christian doctrine is incredible.
- (Premise) Someone believes Christian doctrine.
- Therefore, there is at least one miracle.