Fiction: Bankruptcy in the salvation business; One More Sunday, by John D. MacDonald. New York: Alfred A. Knopf. 311 pp. $15.95 .

AT ECB Enterprises, ''The money rolls in and the visitors roll in'' - along with magazine and newspaper people, politicians, the Internal Revenue Service, and others. ECB owns ''a couple of Gulfstream jets, a couple of small Beechcraft , and a chopper.''

What is this outfit that John D. MacDonald is writing about in his 71st novel , ''One More Sunday'' - a multinational energy company? A big conglomerate in the Fortune 500? Why no, ECB is an acronym for the Eternal Church of the Believer, an enormous and wealthy media church, and quite obviously from Page 1 the kind of organization for which MacDonald has little use.

Donations to ECB arrive in such volume as to allow the accountants to weigh the paper money (''As has been the practice at large busy racetracks for many years,'' MacDonald does not hesitate to observe). The church even accepts Visa and Mastercard. ECB owns a shopping mall, a retirement village, a university, and professional broadcast facilities. It plans to build an enormous hospital complex. Its books are examined daily; the records are on computer. It's an empire.

''One More Sunday'' is built on the model of one of MacDonald's more recent best sellers, ''Condominium'' (1977). There are numerous subplots, lots of action, a DeMille-like cast of characters, and good pacing. But the new novel is much better than ''Condominium'' for the simple reason that it has a stronger central story line.

Roy Owen, an investment consultant from Connecticut, is in Florida to locate his missing wife, who disappeared while researching a story on ECB for Out Front magazine. His search is one of the many interesting subplots in the novel, and Owen is a character refreshingly like you and me.

But ECB and its staff take center stage, and what a sorry collection they are! Mary Margaret, daughter of the church's founder, Matthew Meadows, is a glutton; Joe Deets, reverend-cum-computer whiz, just can't keep his hands off young girls; John Tinker Meadows, the church's leader, is an adulterer suffering what can only be called spiritual burnout; the Rev. Dr. Walter Macy is covetous, ambitious, self-righteous, and crazy; and the list goes on.

Matthew Meadows is suffering from a deadly disease - we learn about that; ECB has a massive computer system - we learn about that, too; we also get a close-up of a small-town believer.

While the Ten Commandments are broken with depressing regularity at ECB, MacDonald's purpose is not to conduct a Dresden-like bombing of evangelists. No, the problem here is scale. The church has gotten too big, too interested in money and creature comfort, and oh so far away from the basic business of salvation.

To make it absolutely clear that we understand that ECB has lost the human touch, MacDonald gives us the Rev. Tom Daniel Birdy, a crude but truly charismatic preacher the church is trying to recruit. After a visit to ECB, Birdy tells them: ''I got into this line of work to tote souls to Jesus! To tote them my own personal self without million-dollar satellites and million-dollar airplanes and five hundred pounds of computer forms. What this whole place does is separate you from your people, and that separates you from God and Jesus Christ.''

''One More Sunday'' affords generous opportunities for MacDonald to mount his own pulpit, and he misses few of them. But the creator of the 20 Travis McGee detective novels knows what he is doing. In fact, he is one of the most skillful writers of popular fiction we have, as this novel amply shows.