Church membership in this country is growing more than twice as fast as the population is increasing, with theologically conservative churches and Roman Catholics growing fastest and mainline Protestants continuing to shrink, but at a slower pace, according to the latest Yearbook of American Churches.

Membership figures for 1981, the most recent statistics available, show an overall increase of 2.69 percent over the previous year, while population growth was estimated at .9 percent.

But Constant H. Jacquet, veteran editor of the Yearbook, warned that the overall figures are somewhat inflated as the result of changes in statistical reporting in two denominations. The Church of God in Christ, headquartered in Memphis, updated its figures for the first time in 16 years, from 425,000 in 1965 to nearly 3.3 million in 1981.

In addition, the Church of Jesus Christ of Latter-day Saints changed its reporting practices to include some 650,000 unbaptized youth as full members.

The Yearbook, published by the National Council of Churches, includes statistics from more than 200 communions in the United States and Canada. The data is based on figures provided by each denomination and overall statistics are therefore not always comparable.

Fastest growth rates were reported by small Pentecostal and holiness churches, such as the Church of God (Cleveland, Tenn.) with 5 percent increase, and the Assemblies of God, with a 3.23 percent growth rate.

Jacquet pointed out that the losses of mainline Protestant churches, such as the Episcopal, United Methodist, and Lutheran, have decreased in recent years to less than 1 percent. An exception was the two Presbyterian churches that reunited earlier this month. A handful of generally conservative congregations of both denominations took advantage of special legislation that permitted them to withdraw from the parent bodies without losing their property.

Theological talks between representatives of Roman Catholic and Oriental Orthodox Church have led to a new agreement on the eucharist.

A joint statement issued by representatives of the two groups, who met in Jamaica, N.Y., earlier this month, confirmed agreement that "the power of the triune God effects the change of bread and wine into the body and blood of Christ in the eucharist."

The statement adds that "the consecration of the elements is effected through Christ," according to a Roman Catholic spokesman.

Involved in the talks with the Roman Catholics were representatives of Ethiopian, Coptic, Syrian and Armenian churches from the Oriental Orthodox side. Future consultations are expected to develop a statement on mixed marriages.

American leaders of the Baha'i faith have appealed to President Reagan to do what he can to prevent further killings of Baha'is in Iran. Last week, the Khoumeni regime hanged six more Baha'i men and 10 women, including three teen-age girls, and church leaders here fear for the lives of some 60 more Baha'is jailed in Shiraz.

Shiite Muslims consider the Baha'i faith, which originated in Iran, a heretical offshot from Islam. Within the past four years, more than 150 Baha'is have been executed, thousands more dismissed from their jobs or made homeless and countless cemeteries and sites holy to the faith have been desecrated. In a protest against these measures, Amnesty International has asserted that Baha'is are being persecuted solely for their religious beliefs.

Legislation that would permit divorced Anglicans to remarry within the church is coming before the Church of England's General Synod next month. But the proposed measure would require a diocesan bishop to conduct a "full and sensitive" inquiry, resembling in some respects the Roman Catholic annulment procedure, into each request for a second marriage before granting permission.

The Washington, D.C. area Council of Churches and the United Methodist Annual Conference of Virginia have developed a plan to distribute blemished or otherwise unmarketable farm produce to the needy in the nation's capital. The first distribution of 40,000 pounds of sweet potatoes, took place Thursday, along with U.S. Department of Agriculture surplus cheese.