Many moons ago — just under a quarter of a century — I covered a major ecumenical event in the Episcopal Diocese of Colorado. I believe it was a festive Divine Eucharist marking the departure of Bishop William C. Frey, as he exited to serve as dean and president of the Trinity School for Ministry.
One of the honored participants in the service was Denver Archbishop J. Francis Stafford, who was a national level figure in Catholic ecumenical efforts (and today is a cardinal serving at the Vatican). It was natural for Stafford to be there, in large part because he had a positive working relationship with the charismatic Frey, who was a traditionalist on key doctrinal issues that affected ecumenical work in public life.
Stafford took part in the first half of the service, but did not formally vest to take part in the Holy Eucharist itself. As the rite moved into the sacramental prayers of the Mass, the Catholic archbishop moved to the side of the auditorium — where a prie dieu had been placed, allowing him to respectfully kneel in solitary prayer.
The symbolism was important: Stafford was there in prayer, but because the Catholic and Anglican churches are not in Communion, with a large “C,” he could not take part in the celebration of the Mass (with female priests, for example) or receive Communion. Stafford was there as a show of unity, to the degree allowed by the doctrines of the two churches.