Is Christianity Testable?

Is Christianity testable? Yes and no. Certainly no in the sense that perceived miracles are sporadic and unforeseeable - but yes in the sense that one's own personal journey is constantly being examined in light of experience. You see, to the outside observer there is a hiatus between the first and third person perceiver, which conflicts with the practice of evidential scrutiny. To the person building the decked successions of experiences, religious belief is a construction of personal narratives and logical examinations with a hard-thought medley of post-hoc experiences to arrive at a set of beliefs and views that sit on a well-balanced fulcrum between our theories and our experiences.

Proclaiming that religious belief is irrational is useless without a simple demarcation as to what constitutes rational and what constitutes irrational - and you'll always find this part lacking. Even the most ardent religious sceptic can't fail to concede that religious faith, while not as accessible as science, amounts to a personal narrative that assimilates much of the same kind of material available to the atheist into his explanatory endeavours. It's no use simply to say that rationality means assenting to empirical evidence, because the relationship between empiricism, theory and narrative involves further explanatory strategies, which themselves tap into issues of metaphysics and ontology, as well as emotional assessments of reality that amount to a whole worldview.  

Patterns that one believes to be God-inspired are always going to be sparse compared with the wide fabric of empiricism available to us - but the human propensity for narration and investment of meaning in those patterns ensures that, as long as one doesn't over-cook one's theorisations, one needn't be constricted only by views and beliefs that can withstand scientific scrutiny. The experiences of Christians that make them Christians are not often experiences that are going to be demonstrable or evidential to non-Christians. I've a long history of examples where atheists have been infuriated by this - but infuriation or not, it happens to be true, which means there will always be an exploratory hiatus for those that can't appreciate this.