The diocese confirmed the death of the 73-year-old the death of the 73-year-old Steyl missionary, which was completely unexpected: "He was neither ill nor had he complained of any health problems."
Rebello celebrated Mass in a parish on Saturday morning after having previously taken part in a 20-kilometre rosary walk.
The diocese described the circumstances of his death: During the sermon, which he had not delivered himself, the bishop had briefly fainted. After members of the congregation had given him water and juice, he was initially well again.
He was still able to receive communion, but then retired to the rectory.
After the mass, Rebello was taken to hospital, where he died.
The diocese did not give a cause of death.
In Botswana, Rebello was known for his commitment to the human rights of migrants and refugees, who mainly come to the country from Burundi, Angola, Zimbabwe and Uganda.
He emphasised the hospitality of his diocese and criticised the conditions in the refugee camps.
Born in Kenya in 1950, he joined the Steyler Missionaries and was ordained a priest in 1977. He initially worked in India and was a priest in Botswana until 1984 before becoming provincial of his order in Kenya.
Further stations as a missionary were Antigua and Barbuda as well as Angola and again India. Rebello returned to Botswana in 2003 and was appointed Bishop of Francistown by Pope Francis on 5 July 2021.
Francistown, which was established as an apostolic vicariate in 1998 and elevated to a diocese in 2017, has always been led by Steyler missionaries.
The Southern African Conference of Bishops (SACBC), to which the Francistown diocese belongs, was saddened by the sudden death and expressed its condolences to the diocese, the Order of the Steyler Missionaries and the family of the deceased.
"May all be comforted by the knowledge that Bishop Rebello was a humble man of strong and exemplary faith who we believe is now in the company of his risen Lord," said SACBC Chairman, Bishop Sithembele Anton Sipuka.