Lucas Oil Stadium, the vast brick home to the Indianapolis Colts, is space custom-built for commotion and cacophony. On Wednesday night, the seats were mostly filled, families having shuffled into rows balancing paper trays of chicken fingers and pizza. But at 7 p.m., the lights went down, and the crowd sat poised in stillness and silence.

Then a voice on the loudspeaker instructed them to kneel, a spotlight beamed to a corner of the floor.

As tens of thousands of people watched, the stadium spotlight shone on a four-foot, elaborate star-shaped gold vessel that contained the simple wafer that Catholics believe becomes the true presence of Jesus Christ when it is consecrated.

At that instant, Camille Anigbogu, 22, from Houston, recalled later, “I profoundly knew that it was God.”

Ms. Anigbogu was among the American Catholics who gathered for an event designed to revive popular fervor for the sacrament of the Holy Communion and to be a “generational moment” for the Church more broadly. It was the first National Eucharistic Congress since the 1940s — brought back in response to American bishops’ dismay that Catholics have been drifting from the ritual at the core of each Mass, and from their faith altogether.