Facing what some have described as its most serious controversy since the Civil War, the General Assembly of the United Presbyterian Church opened its 10-day meeting yesterday in San Diego.

The 2.6 million-member denomination is deeply divided over a recommendation slated to come before the assembly next Monday that would permit the ordination to the church's ministry of acknowledged practicing homosexuals.

The recommendation carries the weight of the majority of a 19-member task force which spent 15 months studying the question at the direction of the denomination's 1976 assembly.

A minority report, signed by five of the 19 task force members, would restrict ordination to the "repentant" homosexual who "finds the power of Christ" either to remain celibate or to undertake a heterosexual marriage.

Both sides quoted scripture as well as psychological and sociological arguments to buttress their point of view.

Church journals have been crammed with letters and articles on the issue for yearly a year.

Denominational leaders have acknowledged serious concern over numerous threats by opponents of the proposal to split from the denomination if the assembly approves ordaining homosexuals.

Particularly unsettling to church leaders is the fact that much of such talk of schism has come from some of the denomination's largest and wealthiest churches.

In the Presbyterian Church, ordinations are performed by regional bodies, called presbyteries. The proposal before the General Assembly would merely establish the policy that otherwise qualified homosexuals might be ordained, but leaves final discretion up to each individual presbytery.

The proposal to ordain homosexuals thus differs sharply from the church's long-standing policy which forbids discrimination against women candidates for the ministry. The church has disciplined clergymen who have failed to uphold this policy.

Unlike some more theologically conservative denominations, the United Presbyterian Church is already on record as supporting civil rights for homoxesuals. The question of ordination is the only issue before the San Diego assembly.