Four out of 10 adults nationwide attended a church or synagogue in a typical week of 1979.

While average attendance has held steady in recent years (41 percent attended on the average in 1978), attendance has declined since 1958, when a peak in church attendance was recorded: 49 percent in a typical week.

Sharp differences are noted by age. Approximately one-third of young adults, 18 to 20 years old, attended church or synagogue weekly in 1979. The proportion drops sharply among adults 21-24, with about one-fourth of them attending church services in an average week.

A gradual increase then begins as young adults marry and raise families, with the proportion increasing steadily as people become older, not tapering off until age 70 or older. A high proportion of the older members of society continue to attend worship services. In fact, a Gallup study of persons 95 and older showed that as many as one-fourth attend weekly.

Persons living in the Middle West (and particularly the west central states) are most likely to attend frequently. Attendance is lowest in the Rocky Mountain and Pacific states.

Formal observance of religion -- at least in terms of attendance at worship services -- does not decline as the level of schooling increases, as is sometimes thought.

About half the Catholics (52 percent) attend weekly, matching the percentage recorded for the 1978 survey. The Protestant figure also remained the same (40 percent), while the percentage for Jews declined from 27 to 20 percent.

The findings emerge from the annual Gallup audit of church attendance. To estimate the average attendance during 1979, surveys of representative samples of the adult population were made in eight selected weeks during the year. A total of more than 12,000 adults, 18 and older, were interviewed in person in more than 300 locations.