Robert De Niro was born to a lapsed Catholic father and an atheist mother who had been raised as a Presbyterian. His parent did not want him to be baptized into the Catholic Church, but his grandparents had him baptized as a Catholic when he was two years old. (His parents were in the middle of a divorce at the time, and he was living with his paternal grandparents.)
Although De Niro has famously portrayed a number of overtly Catholic characters on screen, and he apparently has publicly indicated that he considers himself Catholic, there is no indication that he has ever lived as a practicing Catholic.