The speaker looks at a crowd and sees something like a religious service. The "high mass of people" plays on two meanings: a huge gathering, and the Catholic Mass. Everyone in the crowd becomes part of one ritual, even though they're strangers going about their own business.
The poem picks out two figures: a beggar and a fine lady whose pocket is being picked. Rich and poor, victim and thief all share the same space, and nobody seems to notice the small crime happening in the middle of it. The speaker doesn't scold anyone. The tone is observant, a little wry, and quietly amused.
The last lines make the comparison work. Where a church service has hymns and a sacrament, the crowd has only the "go-along selves" people put on to fit in, and "only mirth" in place of holy communion. The mood is light but slightly uneasy: the ritual is real, but it's built from conformity and laughter rather than anything sacred.