The Southern Baptist Convention was the only Protestant Communion of the nation's 10 largest religious bodies that gained rather than lost members during 1975, according to the latest statistics released by the National Council of Churches.

United Methodist Luthern, Episcopal and Presbyterian churches all lost members.

The Roman Catholic Church, with 48,881,872 members, the country's largest religious body, increased membership four tenths of 1 percent in the reporting period.

The 12,733,124-member Southern Baptist Convention is the second largest body in the nation and had a growth rate of 1.8 per cent.

The 2,336,715-member Church of Jesus Christ of Latter-day Saints (Mormon) had the largest rate of membership gain of any church in the top 10 -- 3 percent. The Mormons are now the 10th largest body in the nation.

The figures reflect membership stawhich complete figures are available.

In contrast to the declining church membership of main-line Protestantism and the near stand-still of Catholic growth, small conservative Protestant groups are showing rapid growth. The Salvation army, with only 384,817 members, had the biggest growth rate --

The trend of church giving established over recent years continued during 1975: members gave more dollars, but not as fast as inflation diminished their value.

Enrollment figures for theological seminaries shot up sharply in both 1974 and 1975, after reaching a plateau in the late 1960s and early 1970s. The number of women in seminaries almost doubled between 1972 and 1975, rising to a total of 6,505.

The number of ordained clergy serving parishes fell by more than 2,500 in 1975. The report does not indicate whether the decline reflects dwindling interest of clergy in parish work or the disappearance of parish positions.

If it is the latter, the growing crop of new clergy emerging from the seminaries in the next few years could be in trouble.