This is the first time in modern Pakistan that the strict code has been in force.
Father Jacob Dogra, of the Diocese of Islamabad-Rawalpindi, which covers the whole province, told the Fides news service: "We are in God's hands. We are very concerned; we never imagined that the bill would be approved by the legislative council."
"Some fundamentalist Muslim leaders in the region have been playing on the sentiments of the people," he added. "Now we must wait and see the consequences of the application of Shariah law for the Christian community."
The Islamabad-Rawalpindi Diocese has a population of about 35 million, including 250,000 Catholics many of whom are committed to education and assistance, with schools of various grades and training institutes and hospitals.
"Christians here live in peace," said Father Dogra. "Except for a few sporadic episodes of violence in the past, Christians work and live in harmony with Muslims, in an atmosphere of friendship and dialogue.
"However, undoubtedly there are some fanatic groups determined to instigate religious hatred. We hope they do not succeed and we are working for this. The adoption of Shariah law is, of course, a reason for concern. We must wait and see the reaction of the ordinary people."
On Monday, legislators voted unanimously to pass a bill to introduce Shariah, or Islamic law, in the province, which is dominated by the Muttahida Majlis-e-Amal coalition of six Muslim parties. The coalition, which swept to power in October, has been criticized by human rights groups who fear a rerun of the Taliban.
Chief Minister Akram Durrani of the province warned: "People who fail to observe Shariah law will have no place in our province."
Shariah applies harsh punishments such as amputation of a limb for stealing, and stoning for adultery. It enforces the compulsory study of the Muslim religion in schools.
Negative reactions to the bill came from the civil society and moderate Muslim groups. The province's 24 mayors, mostly Muslims, will resign in protest, announced Azam Afridi, mayor of Peshawar.
Human rights groups fear most of all for the rights of women. Pervez Rafiq of the All Pakistan Minorities Alliance, comprised of Christians, Hindus and Sikhs, has strongly criticized the move, saying that "religion must not interfere with politics."
Some organizations are considering an appeal to the Federal Court on the grounds that Shariah is contrary to Pakistan's Constitution and the will of its founder, Ali Jinnah, who established the country in 1947 as a secular state guaranteeing the rights of religious minorities.
Religious minorities in Pakistan recall 1999 when they led a successful campaign against the introduction of Shariah, and stopped the federal Parliament from approving an amendment to the Constitution which would have subjected the whole country to the Koran.
There are 3 million Christians in Pakistan, 1.2 million of whom are Catholics. About 96% of the country's 147 million inhabitants are Muslim.