Quoting a dictionary in an argument is not just bad form but completely incorrect. An English dictionary is not an authoritative source for the meaning of words -- it's a record of the usage of words by English speakers and authors. It is a tool that helps a reader survey the possible meanings of the speaker or author. It does not define the meaning of a text.

You would do well to argue against the authors ideas rather than their words.

It can be helpful to enlighten an ignorant person though. Take the bastardization of the word "literally", or even "peruse" if you want a less inflammatory example. If one person uses the word in its original meaning and another interprets that word according to a colloquially bastardized version that essentially has the opposite meaning, I think it's entirely appropriate to quote a dictionary to the misinterpreter and incumbent upon them to interpret the word in its original meaning (if at all the intention is in question).

Bastardizing words, particularly those that have a prior established meaning to a minority community like atheists, is tyranny of the majority. It's not their fault that people don't know what atheist means and interpret the word incorrectly. If you reject that notion then I think you a) endorse a recursive loop where ignorant people never have to learn new vocabulary (or worse, get to make up definitions matching what they want to believe the other person is saying) and b) the bar to exchanging information gets set unreasonably high because speakers have to constantly define terms for their audience. There is some duty to learn and understand in a conversation. If you don't know what the words mean, look them up or don't get upset when corrected. That's how you have a conversation that progresses past semantics.

Since you're arguing with definitions, I feel it worthwhile to point out that these definitions do not necessarily contradict each other. Note the use of the word "especially".