Roman Catholic Bishop Thomas J. Welsh of Arlington yesterday warned Catholics attending outlawed Latin masses and confessions conducted by an itinerant rebel priest that they do so at some peril to their eternal salvation.

"The validity of the sacrament of reconciliation (confession and absolution) is based upon the granting of faculties, or authority, by the local ordinary (bishop) to hear confessions and administer absolution," Bishop Welsh said in a prepared statement.

The Rev. Dan Dolan, of East Meadow, N.Y., a priest of the Society of St. Plus X, heard confessions and conducted the forbidden Tridentine Latin Mass on Sunday for about 200 persons. He said he had not sought Bishop Welsh's permission to officiate in the dioeese, as required by church law, because he didn't recognize the bishop's authority.

A spokesman for the bishop said that the "inescapable conclusion" of the bishop's statement was that in the eyes of the church. Sunday's penitents at the traditionalist rite had not been absolved of their sins. Catholics believe confession and absolution from serius sin is a key element in salvation.

Bishop Welsh also said that "attendance at a Tridentine mass does not fulfill the Sunday obligation of a Catholic" to go to mass.

The Arlington bishop expressed "deep personal sadness that the church which constantly faces difficulties from outside forces must now suffer confusion as a result of misguided zeal and fervor by members of the Society of St. Pius X."

The Society, headed by the dissident French Archbishop Marcel lefebvre, has challenged all the reforms effected by the Second Vatican Council of the Catholic Church. It maintains that by refusing to accept these reforms, the Society, not the established church, is faithful to the traditions of the Catholic faith.

Reacting to claims made by Father Dolan that only the Tridentine mass was the "true mass," Bishop Welsh said, "Mass is celebrated and the sacraments are validly administered by the priest of our diocese and those priest visitors who seek and obtain faculties."

He expressed hope that "the faithful of our diocese will not be caught up in any misunderstanding that may have resulted" from the Tridentine mass, which is expected to be repeated on a monthly basis.

Asked if Bishop, Welsh anticipated any stronger action against Arlington Catholics who attend the forbidden masses, Leonard Reed, diocesan information officer replied: "The action he is taking is the most appropriate one - he's praying hard for them."