After taking his oath, Adityanath said he would focus on development for all communities in a state where Muslims make up almost 20 percent of the population.
The BJP also sent out reassuring signals. Prime Minister Modi tweeted that 'Our sole mission and motive is development. When U.P. [Uttar Pradesh] develops, India develops.'
Another top leader, Venkaiah Naidu said, 'This mandate is against caste politics, religious politics,' Naidu said.
But Adityanath's elevation to the top post in Uttar Pradesh has unsettled many including liberals, opposition parties and political observers.
Calling it a baffling choice, independent political analyst in New Delhi, Ajoy Bose said, "Everybody is a little confused as to why the Prime Minister and party president chose this name."
The discomfiture about Adityanath's appointment arises from the incendiary speeches and remarks laced with anti-Muslim sentiment the Hindu leader, who has been a member of parliament, has made in the past.
He recently praised the first travel ban ordered by U.S. President Donald Trump to block immigration from a group of Muslim-majority countries, saying India needed something similar to tackle terrorism.
He has also said minorities who oppose yoga should leave the country and once compared top Bollywood star Shahrukh Khan to a terrorist. He was reprimanded by election authorities for calling on Hindus to convert 100 Muslims for every Hindu that adopted Islam.
In campaign rallies, he supported the building of a Hindu temple on the site of a 16th-century mosque that was razed in the 1990s, igniting religious riots.
Greater polarization feared
The Times of India newspaper called the selection of the "saffron hardliner" a "defiant assertion" of the BJP's Hindu nationalist credentials.
In a sentiment echoed by many opposition leaders, Congress Party spokesman Manish Tewari, said the BJP's decision to appoint Adityanath was a 'harbinger to greater polarization'.
And although BJP leaders defended Adityanath's appointment, political observers say it would dent the the ruling party's image, which had been boosted hugely by its recent success in regional elections. The BJP's sweeping success in Uttar Pradesh had put the party in a strong position ahead of the 2019 elections.
"A lot of people feel kind of let down," said Bose. "This was a huge victory and had really made Mr. Modi quite invincible in some ways and this was the time really to move on and to tell people that now we are in power in Delhi and in India's largest state and political heartland, we will get onto the modernizing agenda."
Many in the country have thrown their support behind Modi because they see him as a strong leader who can deliver on promises of wiping out corruption, reviving the economy and creating jobs. But they want him to steer away from anything that would hurt India's secular credentials - something that Modi has done by emphasizing that his main agenda is "development for all."
Bose said people will watch carefully to see how Adityanath conducts himself as Chief Minister. "If he keeps away from a divisive agenda, gives a good government, there is some development in U.P. then these concerns will lessen."