Kerala became the first Indian state to challenge the country’s controversial citizenship law before the Supreme Court on Jan. 14 following nationwide protests against the measure, which critics say could turn Indian Muslims into second-class citizens.

The state’s lawsuit argues that the Citizenship Amendment Act, which offers a fast track to Indian citizenship for non-Muslim refugees from three neighboring countries, is “discriminatory” and runs counter to the Indian constitution’s secular principles. Here’s what to know.

What is the Citizenship Amendment Act?

On Dec. 11, Prime Minister Narendra Modi’s ruling Bharatiya Janata Party introduced the Citizenship Amendment Act, which allows Hindu, Parsi, Sikh, Buddhist, Jain and Christian citizens of Afghanistan, Pakistan and Bangladesh who came to India before 2015 to claim Indian citizenship.

Prime Minister Narendra Modi’s ruling Bharatiya Janata Party (BJP) has said that the citizenship law is aimed at protecting persecuted minorities and that it will not affect any Indian citizen. However, critics say the initiative fits into the BJP’s broader anti-Muslim agenda that prioritizes creating a Hindu nation.

Anti-Citizenship Amendment Act protests have rocked the country for weeks, with tens of thousands of Indians taking to the streets. At historically Muslim universities, and in the state of Uttar Pradesh, protesters have faced a violent crackdown. At least 23 people have died since the protests began.

There are also fears around a BJP plan to make a list of all of India’s legal citizens through a National Register of Citizens (NRC) that requires Indians to provide documentary evidence of being in India for decades. A trial run of the exercise already took place in Assam last year and put nearly 2 million people at risk of losing their citizenship. Many residents in Assam, due to displacement and poverty were unable to provide the documents required by the NRC to prove their citizenship dating back 50 years.

India’s Home Minister Amit Shah has repeatedly said he wants to roll out the NRC nationwide, although exact details about how and when the government would do so remain unclear. Critics have expressed concern that the Citizenship Amendment Act would provide an avenue for non-Muslims who cannot prove their citizenship to qualify for the NRC and remain in India, while leaving Muslims at risk.

Assam’s state government has said it plans to build 10 detention centers for those who are denied citizenship even after an appeal. Construction is already underway and a Minister of State for Home Affairs has said all states have been directed to build detention centers. Critics fear that these detention centers may eventually primarily be used to house Muslims. The BJP has denied such claims, saying that such facilities are meant for “foreigners who are in India illegally.”

What does Kerala’s lawsuit say?

It says the law violates several articles of the Indian Constitution, which include “equality before the law” and protections of life and personal liberty and freedom to “profess, practice and propagate religion.” Kerala’s chief minister Pinarayi Vijayan said in a statement on Facebook that the lawsuit is part of the state’s intervention to “protect civil rights.” Vijayan said the citizenship law runs against the Indian constitution’s secular values.

The Kerala lawsuit also questions why the citizenship law does not offer protection to Muslim minority sects, like Ahmaddiyas and Shi’ites, who are also persecuted in these countries. The lawsuit says the class of religious minorities receiving the opportunity to receive a fast-track to citizenship have been “irrationally chosen.”

Why has Kerala filed it now?

At least 60 petitions against the citizenship law have already been filed by various individuals and political parties before the Supreme Court and will likely be heard on Jan 22, according to the Hindustan Times, but Kerala is the first state to take its case to the country’s highest court. “From a constitutional and legal standpoint, a state joining the myriad of other suits (…) adds additional clout,” Manoj Mate, a visiting professor of law at UC Irvine and expert in India’s constitutional law, tells TIME.

Mate notes that Kerala stands out for its commitment to religious pluralism, as well as its long tradition of leftist, progressive, communist politics. Kerala’s legislative assembly previously passed a resolution demanding the repeal of the Citizenship Amendment Act and the chief minister wrote to 11 other chief ministers requesting their intervention against the citizenship law, which he referred to as “fundamentally discriminatory in nature.”

And there is a “strong legal basis” for challenging the Act, Mate says, especially under Article 14 of the Constitution, which notes that, “the State shall not deny to any person equality before the law or the equal protection of the laws within the territory of India.”

Mate says it’s unclear what legitimate rationale exists for expediting the paths to citizenship for some religious minorities paths, but not Muslims. “When you’re treating different groups differently there has to be some reasonable or rational basis for doing it. Here, there doesn’t really seem to be one,” Mate says.”

What is the Supreme Court likely to do?

It’s hard to predict how the Supreme Court will decide the case, Mate notes. The case could prove to be a “defining moment for the court,” he says, especially because a string of judgments over the last three decades have been “more allowing of religious majoritarianism, specifically by the BJP and the Hindu right.”

Earlier in January, India’s top court upheld that indefinite restrictions on communication and movement in the Kashmir Valley were invalid but stopped short of directing the government to end them. Last year, the Supreme court allotted land that would allow the building of a Hindu temple at the site of a mosque demolished by a Hindu nationalist mob in 1992. And bout three years ago, the court declined to reconsider a 1995 ruling that defined Hindutva as a way of life; critics say that the previous judgment could facilitate the domination of BJP’s Hindu ideology.

Despite a massive victory in national elections in May, the BJP has struggled in state polls and the number of state governments that they control has shrunk significantly over the last two years. Kerala’s decision to challenge the law speaks to the divide between states and the central government. “The states are saying that the sovereignty of India’s parliament cannot just walk over the will of the people, of states, or their right to determine things for themselves,” says Manu Bhagavan, a professor at the City University of New York and expert in South Asian history.

“It’s an almost American-style resistance from the states — the kind that Indians really have not seen” for decades, adds Rohit De, an assistant history professor at Yale University and expert in South Asian legal history.

It is possible that even more states could wage a similar legal battle before the Supreme Court, signaling expanded support against the citizenship law, Mate says. Punjab’s advocate general Atul Nanda told The Print that the state is “seriously contemplating” challenging the citizenship law in the Supreme Court. If more states take their case to the country’s top court, Mate says it will signal “a clear schism across the country.”

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Write to Sanya Mansoor at sanya.mansoor@time.com.