When it comes to atheists mocking religious belief - something I have long advocated here - a couple of crucial distinctions must be made. First, mocking someone's religious beliefs is not the same thing as attacking them personally. The distinction is not always easy to spot, but I think it is an important one. Second, not all forms of mockery are equally effective. In fact, forms of mockery that might be perfect for certain believers will fall flat or make the situation worse with others.
The goal of mocking religious belief is simple: provoke thought among all who encounter it. I seriously doubt that one experience of having one's religious beliefs mocked is going to lead any believer to de-convert. The best we can hope for is to provoke thought, and we should probably view effective mockery as having a small, cumulative impact. This is part of why even fairly subtle forms of mockery can be effective.
To understand the difference between mocking one's religious belief and personal attacks, we need to examine the target and the intent. The sort of mockery I advocate focuses on the beliefs rather than the person and is intended to provoke thought among those who encounter it. Personal attacks, on the other hand, focus on the person and are generally intended to inflict emotional pain.
To be effective, mockery must be tailored to the person and the situation. This is where the skill comes into play. To a close Christian friend, I might say something like, "How can you really believe that? I always thought you were much too smart for that sort of thing." I wouldn't say this to someone I barely knew. In that case, I'd be far more likely to talk about myself. Faced with someone saying asking me to pray, I might say, "No thanks. I outgrew that a long time ago."
There is a legitimate concern that some forms of mockery, especially those directed toward strangers, may provoke defensive reactions. I do not dispute this, and I also recognize that some people will react defensively no matter how they are approached. This is part of why mockery cannot be our only strategy. At the same time, I am convinced that it is an effective tool for provoking thought.