VATICAN by Malachi Martin (Harper & Row: $18.95). Malachi Martin, the author of "Vatican," is one of those writers who can't bear to leave anything out. During the entire course of this great lumbering novel, which is set against the background of the Holy See's involvement in world affairs from the mid-1940s to the present and slightly beyond, he devotes most of his time instructing, explaining and lecturing to us. He wants to make sure that we understand exactly how the Church works and why, even at the risk of suffocating us with facts.
Some writers, like James Michener and Irving Stone, can get away with this technique, because they manage also occasionally to create credible and interesting characters. Unfortunately, Martin is not in their class. The people in his roman a cupola are cardboard cutouts, who think, talk and act exactly like characters in a TV miniseries, and his protagonist, a young American priest named Richard Lansing, could have been written for Richard Chamberlain; he is painfully sincere, intelligent, and safely passionate. The tale he stars in will seem no more credible on TV, but it will be trashier and a lot more fun.