For the first time in six years, the average amount that members of 10 major Protestant denominations gave to their churches failed to keep pace with inflation.

Per capita giving to the denomination rose only 9.9 percent in 1979, compared with an inflation rate of 13 percent. The churches suffered additionally because they had a net membership loss, which made their adjusted per capita increase only 8.8 percent, according to statistics just released by the National Council of Churches.

Constant H. Jacquet of the NCC Office of Research, Evaluation and Planning, pointed out that regional and national church offices and national and world councils of churches are in real trouble financially. With total churches taking in less and with their expenses increasing, fewer dollars are handed on to the larger bodies, Jacquet said. Churches in the study sample included the American Baptist church, the three major Lutheran bodies, two Presbyterian denominations, the United Church of Christ, United Methodist, Episcopal and Christian Churches.

Of the 44 denominations included in the NCC's broader statistical study, highest per capita giving was registered by the Independent Fundamental Churches of America, which received an average of $732 per member during 1979. The Seventh-day Adventists, with a reported average contribution of $677 per member, were next highest:

The nation's two largest Presbyterian bodies, the United Presbyterian Church in the U.S.A. and the southern-based Presbyterian Church U.S., could be reunited within the next few years, if a plan worked out by leaders of the two groups is accepted. The proposal to mend the rift brought by the Civil War will be voted on by the national assemblies of the two denominations next year and, if approved, forwarded to regional presbyteries for ratification. The new united church tentatively would be called the Presbyterian Church (USA).

One of the sticking points in the process is the United Presbyterian Church's requirement that women must be elected to local church offices. A compromise agreement preserves this provision, adopted three years ago, but would include a conscience clause under which congregations could be exempted. Both churches permit ordination of women to the ministry.

The plan also would only allow dissenting congregations of the Southern church to withdraw and retain their property. They must wait three years after the reunion, however, before beginning negotiations for secession.

Efforts to unite the two Presbyterian bodies, whose combined membership would excede 3.3 million, have been under way for at least two decades. Locally, as in several other major cities where both denominations are strong, the reunion already has been effected through the creation of the National Capital Union Presbytery.

A study by the Anti-Defamation League of B'nai B'rith maintains that recent court rulings and actions of local governments have tended to weaken the constitutional guarantees of separation of church and state. High on the list of evidence cited to support the finding was the introduction in a number of states of voluntary prayer in public schools. Such programs, said Kenneth J. Bialkin, chairman of the ADL executive committee "are not really voluntary because children who may not wish to participate feel pressure from their classmates to join in."

The ADL also criticized Bible literature courses in public schools and court rulings approving the celebration in schools of religious holidays such as Christmas. Also cited was the refusal of Fairfax County schools last spring to honor the request of two Jewish girls, the top-ranked scholars of their class, to reschedule high school graduation exercises, which fell on the Sabbath.

The attorney general of California has dropped investigations of 12 religious groups, including the Worldwide Church of God, because a new state law bars him from investigating tax exempt religious organizations. In adopting the new law, the legislature refused his request to include a clause that would have preserved actions already filed. Nearly two years ago, the state charged that Herbert W. Armstrong, founder of the Worldwide Church of God, and the church treasurer, Stanley Radar, had siphoned off millions of dollars from church funds for personal use. The state put the church into receivership for seven weeks.

A U.S. judge in New Hampshire has ordered a new trial in the case of a man who won a $30,000 judgment in a lower court suit in which he charged that the Unification Church had "deprived him of his parental right" by recruiting his daughter when she was a minor.