By Phil Stewart

VATICAN CITY (Reuters) - Pope Benedict, in a decree issued on Saturday, authorized wider use of the old Latin Mass and told the world's 1.1 billion Roman Catholics that his nod to Church traditionalists was nothing to be afraid of.

Jewish groups expressed concern because the decree revived prayers that say Jews are blind to the Christian truth and urge Catholics to pray for their conversion.

One senior figure of a U.S.-based Jewish rights group called the language "insensitive ... insulting" and expressed fears the decree could harm historic reconciliation.

In a letter to bishops, the German-born Pontiff rejected criticism within the Church that his long-awaited move could split Catholics and roll back the clock on reforms introduced in the 1960s, and which are opposed by many traditionalists.

The Second Vatican Council (1962-1965) replaced Latin with local languages in the liturgy, reached out to other religions and struck out texts that Jews found particularly offensive.

"This fear is unfounded," the Pope wrote. "What earlier generations held as sacred, remains sacred and great for us too, and it cannot be, all of a sudden, entirely forbidden or even considered harmful."