Impish defense of Christianity; The End of Christendom, by Malcolm Muggeridge. Grand Rapids: Eerdmans. $2.50 (paperback).

This little book propounds a not very original but still very useful theme. It is summed up in the titles of the two lectures that make up the book's 62 pages: "The End of Christendom" -- "But Not of Christ." In other words, the usurper is dead; long live the exiled king.

One may safely assume that anything Malcolm Muggeridge writes (or speaks) will be lively and provocative -- not to say crotchety, captious, dogmatic, even irascible. It is easy for the loftier sort of intellectuals to dismiss him in his present state of embattled grace as the poor man's C. S. Lewis. But he still has good things to say in an elderly impish way for not too critical people anxiously holding to Christian values in an age of mass religious illiteracy and televised junk-food religion.

The founder of Christianity, writes Mr. Muggeridge, was Christ, whereas the founder of Christendom was Constantine.Therefore it is not Christ's Christianity that today is floundering in a sea of materialism, but the Christendom already condemned by Jesus when he announced that his kingdom was not of this world. a Christianity that seeks popularity in a polluted world is, the author holds, already polluted by the world.

This is a useful observation, if an oversimplified one. What it misses is any possibility of the redemption of society by the same kind of radical spirituality that cast out demons and raised the dead at its first appearing.

But such a judgment gets into deeper waters than so slight a book merits. No one is likely to mistake Mr. Muggeridge for a major prophet, but he is nonetheless a shrewd and often engaging Christian journalist, as when he writes:

"A strange thing I have observed over many years in this business of news gathering and news presentation is that by some infallible process media people always manage to miss the most important thing. It's almost as though there were some built-in propensity to do this. In moments of humility, I realize that if I had been correspondent in the Holy Land at the time of our Lord's ministry, I should almost certainly have spent my time knocking about with the entourage of Pontius Pilate, finding out what the Sanhedrin was up to, and lurking around Herod's court with the hope of signing up Salome to write her memoirs exclusively."

To which he adds a sad comment on a media-mesmerized society: "Ironically enough, as the dramatization of the public scene gains impetus, so we move farther and farther from the reality of things and become mor e and more preoccupied with fantasy."