When did it be legal to be a Catholic again?

Answer 1

Assuming that you are talking about England, the Catholic Emancipation Act passed in 1829:

from Wikipedia:

The Roman Catholic Relief Act 1829, passed by Parliament in 1829, was the culmination of the process of Catholic Emancipation throughout Britain. In Ireland it repealed the Test Act 1673 and the remaining Penal Laws which had been in force since the passing of the Disenfranchising Act of the Irish Parliament of 1728. Its passage followed a vigorous campaign on the issue by Irish lawyer Daniel O'Connell. O'Connell had firm support from the Prime Minister, the Duke of Wellington, as well as from the Whigs and liberal Tories.

The Act permitted members of the Catholic Church to sit in the parliament at Westminster. O'Connell had won a seat in a by-election for Clare in 1828 against an Anglican. Under the then extant penal law, O'Connell as a Roman Catholic, was forbidden to take his seat in Parliament. Sir Robert Peel, the Home Secretary, who had until then always opposed emancipation (and had, in 1815, challenged O'Connell to a duel) concluded: "though emancipation was a great danger, civil strife was a greater danger." Fearing a revolution in Ireland, Peel drew up the Catholic Relief Bill and guided it through the House of Commons. To overcome the vehement opposition of both the House of Lords and King George IV, the Duke of Wellington worked tirelessly to ensure passage in the House of Lords, and threatened to resign as Prime Minister if the King did not give Royal Assent.