A leading Catholic priest has spoken out against the Papua New Guinea parliament's move to recognise Christianity in its constitution.

Father Giorgio Licini, who is completing a two-term Caritas service as the General Secretary of the Catholic Bishops Conference of Papua New Guinea and Solomon Islands, said impressions Papua New Guinean life is set to improve by the parliament declaring Christianity that includes Catholicism the nation's official religion is an "illusion".

"To create, nowadays, in the 21st century, a Christian confessional state seems a little bit anachronistic," he said.

Fr Licini said the Catholic Church, which accounts for 27 per cent of the nation's followers and the largest of all faiths of Christianity in Papua New Guinea, would have preferred no constitutional change.

He believes the government's motion had comprehensive support from across the benches in somewhat of a conscious vote is only a "cosmetic" change that "will not have a real impact" on the daily lives of an estimated 96 per cent of the country's Christians.

"Papua New Guinean society will remain basically what it is," he added.

The parliament in Port Moresby last week amended the nation's constitution, introducing a religious declaration in its preamble before giving all forms of collective Christianity faiths official state status.

The amendment declared: "(We) acknowledge and declare God, the Father; Jesus Christ, the Son; and Holy Spirit, as our Creator and Sustainer of the entire universe and the source of our powers and authorities, delegated to the people and all persons within the geographical jurisdiction of Papua New Guinea."

The resolution overwhelmingly passed in the parliament 80 votes in favour to four against.

Christianity will also be reflected in the fifth goal of the constitution, while the Holy Bible will also be recognised as a national symbol.

Fr Licini said the preamble of the 1975 Constitution when Papua New Guinea accepted the independence call from Australia already acknowledged the nation's Christian heritage.

"This manoeuvre may offer the impression or the illusion that things will improve for this country," he said, "that the way of behaving, the economic situation, the culture may become more solid, but that is an illusion".

Fr Licini also believed secular cultures and values are scaring many followers in the nation, including the recognition and increasing acceptance of the gay "rainbow" community.

"They see themselves as next to Indonesia, which is Muslim, they see themselves next to Australia and also of New Zealand, which are increasingly secular countries, the Pacific heritage is fading, so the question is, who are we?" he said.

"It looks like a Christian heritage and tradition, and (also) values of the churches, they offer an opportunity to ground on them a cultural identity."

Prime Minister James Marape, a vocal proponent for the latest religious amendment, has said he was delighted with the outcome.

Mr Marape was quoted saying the amendment "reflects, in the highest form" the role Christian churches have played in the development of the country.

The Prime Minister has maintained that "it is not an operational law," but a stance that remains rather more symbolic, which he hopes will unite the people of Papua New Guinea.