Italy's most senior Catholic bishop Monsignor Nunzio Galantino was speaking after the lower Chamber of Deputies voted 372-51 with 99 abstentions to approve same-sex partnership legislation, which has already been passed in February by the upper Senate.
The legislation grants Italian gay and lesbian couples some of the legal rights which married couples already have.
The new law is not marriage and it is still illegal for gay couples in Italy to adopt children.
The Vatican has also reaffirmed its position that marriage is a lifelong commitment between a man and a woman.
Italy is the last of the European Union's 28 nations to grant legal recognition to civil unions.
Gabriele Piazzoni, national secretary for gay rights group Arcigay, said: "It is lacking full equality, which is what we were asking for.
"But still, this is a crucial moment."
Italy's premier Matteo Renzi said on Facebook: "Today is a day of celebration for so many. For those who finally feel recognised."
