REYKJAVIK, Iceland (AP) _ On a lava field where Christianity was proclaimed in Iceland nearly a thousand years ago, Pope John Paul II appealed Saturday for Christian unity and primacy of moral values.

The pope focused on the weakening of family life and reconciliation among Christians in this country where 2,300 of the 244,000 people are Roman Catholic, and where more people play than worship on Sundays.

Iceland was the second stop on the pope's five-nation visit to Scandinavia, a Lutheran stronghold where the 16th century Protesant Reformation swept away Catholicism and the current affluent lifestyle has eroded traditional values of the church.

He chose an ecumenical service at Thingvellir - where the world's oldest parliament was founded in 930 and Christianity was proclaimed the national religion over paganism in 1000 - to urge all Christians to ''heal the wounds of division and re-establish unity.''

He spoke in English peppered with a few words of Icelandic to the congregation of several hundred seated against a backdrop of spectacular snow- covered mountains:

''It is essential to recover an awareness of the primacy of moral values, to reflect on the ultimate meaning of life and its transcendent destiny ...

''The fact is that families are facing new and serious pressures which can only be met by a renewed and deeper respect for life and love.''

Peter Sigurgeirsson, bishop of Iceland and leader of the Lutheran church, said he didn't want ''to belittle'' issues dividing the Christian churches.

''But in actual fact, the importance of the things that separate dwindles when viewed in the light of that which unites. It is the faith in Christ which unites us,'' he said.

Some of Iceland's 17 women Lutheran ministers boycotted the service. The Rev. Hanna Maria Petursdottir said they were protesting the Catholic Church's refusal to allow women into the priesthood.

By Icelandic standards, security was tight for the papal visit. The communist newspaper Thjodviljinn noted he had six bodyguards and that the entire Reykjavik police force was mobilized.

John Paul flew to Iceland from Tromsoe, Norway, where he extolled marriage and strong family values before fewer than 1,000 people. It was one of the smallest open-air Masses of his 42 foreign trips. Like Icelanders, many people live together in Norway without getting married.

Police at Tromsoe detained four people - two fundamentalist women protesting ecumenism and two men criticizing the Vatican's human rights record.

When the pontiff arrived at Keflavik Airport for the 24-hour stop in Iceland, Prime Minister Steingrimur Hermannsson welcomed him ''as a true servant of the God in whom we believe, regardless of the church to which we may belong.''

06-03-89 1533EDT