Why is the Question Protected?
Though not all users are able to see all activity on this question, there are actually fifteen submitted answers which have been proposed, including six deleted and one with a large number of downvotes. Questions with this level of continued activity should be protected, to protect against low-quality contributions. It's appropriate for a moderator to protect any question in this kind of instance.
Why was this Answer Deleted?
As has been discussed in the comments, this answer makes no mention of the source passage the Question is focused on, and imputes its answer through theology and other passages only. It's appropriate to delete such answers, and then if they are revised to bring them on-topic, they can be undeleted.
Sometimes people get confused about Questions and Answers being treated differently. When users submit Questions which are off-topic, they are usually Closed, and then re-opened if they are revised in line with the site guidelines. Answers, however, cannot be 'closed', and so the only recourses available are comments, flags and deletions. For those unable to see Deleted Answers, this can appear alarming. However, deletions are usually accompanied by a comment to the user, which in this case was both kind and courteous.
Is there an anti-Trinitarian bias on this Question?
Probably not. The top answer is a non-Trinitarian answer, and only has two downvotes on it. Given that the deleted answer doesn't fit the site guidelines, it's easy to understand why it would be removed without needing to look for additional reasons.