Roman Catholic Bishop Paul S. Loverde will preside at an evening Mass on Thursday at George Mason University's Patriot Center in celebration of the Arlington Diocese's 25th anniversary.

About 4,000 people are expected at the 7:30 p.m. Mass that Loverde will con-celebrate with about 100 priests, including the diocese's first leader, Bishop Thomas J. Welsh, and Bishop Walter F. Sullivan of Richmond.

Before the Mass, which is open to the public, about 40 priests will be available in quiet corners of the Patriot Center's vestibule to hear confessions. Each of the diocese's 65 parishes and its three Catholic high schools will carry a banner in a pre-Mass procession.

"There's enthusiasm that we've reached this milestone of 25 years because many of us remember when [the diocese] was formed," said the Rev. Michael Bazan, pastor of St. John the Evangelist Catholic Church in Warrenton, who is also vice chairman of the event's planning committee.

People, he added, "are kind of excited that we've come this far."

The Arlington Diocese was founded Aug. 13, 1974, with Welsh's installation at St. Thomas More Church, now the diocesan cathedral. Until then, all Catholics in Virginia were under the Richmond Diocese. The new diocese began with 136,023 members and 50 parishes. Today, it has 336,000 members and 65 parishes, including All Saints in Manassas, which is the diocese's largest with 5,000 families.

The diocese also has had an infusion of Catholics from other countries as immigration has swelled Northern Virginia's population. Many Vietnamese Catholics, a lot of whom arrived as war refugees, worship at the Holy Martyrs of Vietnam, which opened in 1979. More recently, the diocese's Hispanic population has dramatically increased, with St. Anthony of Padua in Falls Church becoming the largest Hispanic parish in the diocese.

Welsh was succeeded in 1983 by Bishop John R. Keating, who led the diocese until his death in Rome in March 1998. A year later, Loverde, former bishop of Ogdensburg, N.Y., was installed as Keating's successor.

The diocese, which includes the cities of Alexandria, Fairfax, Falls Church, Fredericksburg and Manassas, stretches to Winchester in the west and Kilmarnock in the south. More than 50 percent of its members live in Fairfax County.