Not such a grand entrance at Trinity

It's daunting for contemporary artists to take on a project in an architectural masterpiece, especially when that building has famous examples of art in the same medium.

That was the challenge faced by Alexander Beleschenko and Raffaella Sirtoli Schnell, charged with making walls of glass for the Undercroft of Trinity Church, the Copley Square monument that already boasted a stellar collection of stained glass in the main part of the church by such masters as John La Farge and the pre-Raphaelite painter Edward Burne-Jones. Their biblical scenes are backlit by natural light -- which is the only contribution the outside world makes to this glorious interior.

Stained glass was traditionally used to seal off a spiritual space from the mundane, to encourage contemplation and prayer. Its effect on Gothic-era worshipers, long before the advent of artificial light, must have been staggering. And when much of the population of Europe was still illiterate, the stories these windows told were a way to teach the Bible. The glass had to be legible.

Beleschenko's and Schnell's doors aren't. The two pairs of doors, each a wall of glass 32 feet long and 9 feet wide, are painted and etched with images and words. One immediate problem is that although you can get right up to them, it's almost impossible to read some of the words. And even at a distance, it's very hard to decipher images constructed out of painterly dots and dashes, a style that's somewhere between pointillism and Roy Lichtenstein.

Identifying the images is akin to finding the protagonist in the children's book ''Where's Waldo?"

The doors can be viewed from either side, which means that some of the words are in reverse, which makes you feel as if you're looking from backstage rather than out front. The transparency and translucency of the doors also allows you to see through them, into the freshly excavated tri-part area that's the new home of the church's bookstore and spaces for meetings and events. Approaching the first set of doors, you also see into a meeting room that competes for your eye and adds to the visual overload and confusion.

The first doors are more about the secular; the second, about the sacred, although there's not much traditional Christian iconography in either. In the first, there's an urban skyline at the bottom, a large tree on one side, hands reaching out to each other from the corners, and a giant red compass tracing an arc in the center. God as architect of the universe is one way to read the compass.

The second set of glass doors is a bit clearer. A brilliant blue unifies much of the imagery but also obscures it. A church staff member had to trace the outline of an open book with her hand before I could see it. I never did succeed in making out the hand holding a chalice. The rays streaming down in a fan shape are obvious, though, familiar symbols of God looking after his creation.

There are plenty of stellar 20th- and now 21st-century examples of art glass in religious settings. Chagall and Matisse made glorious contributions to the field. In Boston, artists Howard Ben Tre and David Small created a marvelous glass and bronze piece for the Hall of Ideas that opened in 2002 in the Mary Baker Eddy Library, part of the Christian Science complex. Their work is a fountain of words, quotes from history's great thinkers, in the form of projections that bubble up from the water: Put your hands in and the words flow right over you.

The most riveting use of glass I've seen in a spiritual setting, though, is at Coventry Cathedral in England. After it was decimated by bombs in World War II, church officials decided to let the ruins remain as a reminder of what happened and to build a new cathedral next to them. Stained glass is used abundantly and adventurously inside the new building.

But it's the great 70-foot-high wall that separates it from the ruins that is the heart-stopper. Clear glass in horizontal bands, etched with rows of trumpeting, joyous angels alternating with more sober prophets, this visually simple masterpiece by artist John Hutton allows you to turn one way to see the tragedy of the past and the other to look into the blazing hope of the present.