I think it means "without bite" rather than "without teeth", which is not quite the same thing. I don't think it's saying that they lacked power, the means of asserting themselves, but that they lacked the will to do so. The context refers to the "bloque cristiano" as "indeciso y temeroso", in contrast to the pagan minority, which was "más firme y compuesta". It's about their character rather than their situation. In principle, if they were in the majority, they had the means to prevail, but they failed to do so.

Actually I think the metaphor, "sin mordiente", refers not to biting literally with teeth but to the noun "mordiente" meaning a mordant, an acidic fixer in dyeing or tanning. They lacked "bite" in that sense. I find "sin mordiente" online applied to a boring and unadventurous performance by a rock group, a poor performance by a footballer: that sort of thing.

I've offered three possibilities along these lines; I hope that won't seem like cheating, but it's very difficult to choose and ultimately it's a matter of personal taste.