The verification principle was developed by the Vienna Circle, a group of philosophers who organised themselves in Vienna, Austria, and forms the basis for logical positivism. Its chief proponent, A. J. Ayer, popularised it through his words Language, Truth, and Logic and The Problem of Knowledge, and it forms a comparatively easy-to-grasp principle which can be readily applied to religious language and the problem of meaning in language.
The verification principle is based on the idea that one must be capable of proving a concept for it to be taken seriously; or rather, for it to hold any meaning at all. Assertions must be, in principle, verifiable either through empirical observation, or true of themselves, in order for them to be be meaningful. For example, the statement "All bachelors are men" must be true, for the very definition of a bachelor is an unmarried man. Being true of itself, it is meaningful. If I held up a duck and stated "This is a bird", this would likewise be meaningful, as it can be clearly proven that this is the case.
If we examine religious language, however, then we see how the verification principle can render ideas to be meaningless. The statement "God is love", for example, cannot be empirically proven. God, according to the Judeo-Christian understanding, is taken to be a transcendent being who exists outside the material world. We could never empirically prove His existence, and God or His love are not true of themselves. Therefore, the statement "God is love" is meaningless.
The effect of this renders a great deal of philosophical and ethical thought by the wayside. If these ideas are meaningless, then they are neither true nor false, the Logical Positivists argued, and are not to be taken seriously. With the simple task of determining whether we can empirically prove a statement, much of our previous concepts are rendered wholly redundant.
Logical positivism faced many criticisms. One of those which rose to prominence was Karl Popper's concept of falsification, wherein one should seek ways of proving their ideas wrong in order to justify them. One area in particular in which these two schools of thought differed was with 'positive universal' statements. A good example of one of these would be the statement "all water boils at 100°C". This statement could easily be falsified, by finding water which does not boil at that temperature, but how could one verify it? We could never empirically prove that all water which has ever existed boiled at the same temperature.
Ayer met this point in Language, Truth and Logic, presenting the idea of there being two forms of verification: 'strong' and 'weak'. 'Strong verification' is the same as above, but 'weak verification' differs from this on one key point. If a statement is not verifiable in practice, but is in principle, then Ayer contended that under weak verification it was still meaningful.
Further criticism came from John Hick and Keith Ward. Hick believed that, contrary to the Vienna Circle, religious statements could well be verified - but not in this life. In the afterlife, we would be capable of empirically verifying statements about it, and thus these statements were meaningful as we could eventually verify them. Ward, too, contended that statements such as "God exists" were also verifiable, as there was one being which certainly could verify them - God himself.