A new condo complex in Boston consists of a glassy modern building that appears to be jammed into the remnants of an abandoned church, as if the church is the package, the condos the contents. You’re confronted by the embrace, unembarrassed, of two radically different styles and eras of architecture in one building, something more commonly seen today in Europe than in the U.S.

The complex is called Lucas (nobody ever seems to say the Lucas). At first, it raises doubts whether this is the kind of aggressive architecture that belongs in a historic neighborhood. But the more you study it, the more sense it makes for this particular site and situation.

Lucas stands in Boston’s beloved South End, a neighborhood of 19th-century brick townhouses. The 1874 structure of richly textured granite and puddingstone was the home of the German Trinity Catholic Church until a decade ago, when it was abandoned, taken by the city, and eventually acquired by the developers, New Boston Ventures. To create Lucas, the Boston architects Finegold Alexander gutted the badly deteriorated interior and restored the exterior walls. They thus created a masonry shell, which they refilled with eight stories of condos, four floors of which rise above the old roofline.

The old and new couldn’t be more different. The church is a Gothic pile designed by the amazing Patrick Keeley, who migrated from Ireland at age 25 and went on to design, sometimes with collaborators, more than 500 Catholic churches in the eastern U.S. and Canada. The new condos, by contrast, are a contemporary box of bright glass and black steel.

Two strong design moves make Lucas work. The architects create a relationship between the past and present by placing the steel columns so that each new column reads as an upward extension of the church’s stone structure. You can easily recognize the load-bearing steel as a modernist version of flying buttresses. Trinity and Lucas are members of one tectonic family, even if they’re not members of the same generation.

The second strong move was a decision to completely restore the entire church shell. Failing masonry has been repaired or replaced, and, when needed, new granite window surrounds have been custom cut, with every original opening retained. The church has been retailored for new inhabitants, not discarded.

What couldn’t be recreated were the religious motifs, especially the stained-glass windows. With their Christian iconography, they were removed at the behest of the Catholic hierarchy when Trinity was deconsecrated, and replaced with clear glass.

Lucas contains 33 residential condos, with prices ranging up to about $3.5 million for the upper-floor condos, which run to some 2,750 square feet. In Boston’s hot real-estate market, all the units were quickly sold, most of them before construction ended earlier this year.

The design of Lucas was a response to its particular circumstances. The site is at an extreme edge of the South End. Only a block away, Interstate I-90 barrels brutally through the city. The surroundings are largely parking garages and yet-to-be-redeveloped vacant lots. To have any presence here, Lucas needed to be bold. This part of the South End is designated as a “Historic Protection Area,” a mild level of landmarking. The Lucas design needed no special permissions, but any proposal to demolish the church would have raised immediate red flags.

Economics, of course, shaped much of the design. The church interior wasn’t big enough to contain the number of condos needed for economic feasibility. More volume had to be added somewhere, but the church already filled its property so tightly that the only way was up.

Interior design at Lucas doesn’t amount to much. There’s little public space. Wolf in Sheep, a design firm, provided a gloomy palette of neutral grays for the corridors and a range of off-whites for condo interiors, avoiding shapes and colors that might upstage the rich exterior of stone and steel. For units at the upper floors, the best decor is the view out over the city, as framed by the tall, church-scale windows. Another virtue was unanticipated: fitting the condos around the church’s many irregularities made for a pleasing variety of floor plans.

Lucas is a building that looks a little weird initially but turns out to be the product of common sense: preserving the shell of the church enriches the city with a memory of its past; preservation puts less pressure on the environment than you’d ever get from a conventional campaign of demolition and reconstruction; and Lucas isn’t afraid to look exactly like what it is. This is a confident, legible, individual chunk of architecture. By banging together two eras and two kinds of construction, it subtly celebrates the diversity of the neighborhood and the city.