In a letter sent to Senators on October 31, three Roman Catholic bishops explained their opposition to the Employment Non-Discrimination Act (ENDA), which seeks to prohibit workplace discrimination on the basis of sexual orientation and gender identity.

On Monday, ENDA cleared a critical hurdle in the Senate. However, the bill faces a steep incline in the Republican-controlled House.

(Related: Gay protections bill ENDA clears cloture hurdle in Senate.)

“All people are created in the image and likeness of God and thus possess an innate human dignity that must be acknowledged and respected by other persons and by law,” the bishops wrote. “Furthermore, 'work,' as Pope Francis recently said, 'is fundamental to that dignity.' Thus the Catholic Church has consistently stood with workers in this country and continues to oppose unjust discrimination in the workplace.”

However, the bishops added that the church could not support ENDA because it rejects a biological basis of gender, equates sexual orientation with race, undermines marriage and threatens religious liberty.

“Based on experience in state courts, it is likely that ENDA would be invoked by federal courts to support the claim that, as a matter of federal constitutional right, marriage must be redefined to include two persons of the same sex,” the letter reads in part.

The letter is signed by three chairmen of the United States Conference of Catholic Bishops (USCCB): Bishop Stephen Blaire of Stockton, California, Archbishop Salvatore Cordileone of San Francisco, and Archbishop William Lori of Baltimore.

The bishops did not mention Pope Francis' recent call for the Catholic Church to pay less attention to its rules on abortion, same-sex marriage and contraception.

(Related: Pope Francis says Catholic Church should welcome all, gays included.)