The Rev. Dr. Avery Post, 52, of Boston, was elected president here yesterday of the 1.8-million-member United Church of Christ.

Post, who has headed the church's largest regional conference - massachusetts - for the past seven years, pledge to continue steering the activist denomination on its present course. He succeeds the Rev. Dr. Joseph Evans, who was elected last year to till the unexpired term of the late Rev. Dr. Robert V. Moss.

In his acceptance speech, Post said his first act, even before he takes office Sept. 30, would be to make an official visit to the Rev. Ben Chavis, imprisoned in McCain, N.C. Chavis and eight other black men and one white woman who have come to be known as the Wilmington 10 were convicted in 1972 of arson and conspiracy to assault firemen during February, 1971, racral disturbances in Wilmington, N.C.

Chavis was on the UCC national staff at the time of the incident. The church, maintaining that he and the others were innocent, financed an unsuccessful appeal of the group's controversial conviction.

Delegates to the Biennial General Senate of the UCC continued working their way through a welter of resolutions and position papers on social issued ranging from human sexuality to world disarmament.

TDuring their 14-hour meeting day, the nearest acknowledgment the 703 delegates made to the 4th of July holiday was a resolution of "heartfelt gratitude" for freedom of religion in the United States.

As present-day descendants of some of the nation's earliest settlers who came here seeking the right to worship as they pleased. UCC members continued to be mindful of those religious liberties.

The UCC is the result of a merger 29 years ago between the Congregational Christian Church, rooted in the Pilgrims and the Puritans, and the Evangelical and Reformed Church, which came out of a German and Swiss tradition of the Protestant Refornation.

The UCC's sensitivty to constitutional guarantees of freedom also was reflected in apronouncement deploring "gratuitous violence and the exploitation of sex" on telvision.

While resolving to combat such exploitative broadcasting practices, the pronouncement adopted by the senate said such action should be consistent with "freedom of the press, freedom of expression and freedom of religion, not just because these rights are guaranteed by the Constitution, but because if our society allows itself to be so fettered, it no longer will be open to the new promptings which come. Christians believe, from the living spirit of God."

A senate resolution on the aging called for a church campaign "to combat negative media images" of elderly persons and directed the church's mission boards to launch a ministry to "isolated elderly."

Other resolutions encouraged church members and agencies to pressure American corporations and banks to withdraw investments from South Africa to combat for "universal, accessible and comprehensive health services" in this country, and urged that controversial religious cults be included under constitutional guarantees of religious freedom.