I was recently asked by a skeptic to explain the meaning of "necessary" in the proposition, "God has necessary existence." He held to a non-cognitivist position on the matter, claiming that it is meaningless to assert that anything has necessary existence. This is a very important distinction: he was not merely claiming that nothing has necessary existence, but that the very notion of necessary existence is literally without meaning.
What I pointed out is likely already apparent to most - namely, that if it is meaningful to speak of contingent existence, then it is equally meaningful to speak of necessary (non-contingent) existence. Nobody in their right mind would doubt that there are things that exist, but can also fail to exist.  Trees, planets, human beings, etc., are all contingent beings. If "contingent" is meaningful, then so is its negation, just as not-blue is the meaningful negation of blue.
This reminds me of how essential a sound grasp of apophatic (negative) theology is for the believer. We know the nature of God by knowing what He is not, as opposed to observing God in the same way we observe contingent things. God is said to be necessary because He is non-contingent, timeless because He is non-temporal, immutable because He is not subject to change, and so forth.
On the other hand, this is what makes the Christian doctrine of the Incarnation so scandalous to the non-Christian. How could the immutable God take on a mutable human nature? We could delve into the dual-nature of Christ, but for now I wish only to stress the need for the believer to answer the non-cognitivist's contention that religious language is meaningless. It's easy to answer, but unfortunately non-cognitivism has taken on what I consider to be a last resort in order to resist belief in God.
 It's funny to note that a denial of this would require that everything has necessary existence, which is what the non-cognitivist denies is even meaningful to begin with.