Where It All Goes Down
The setting is so important to this poem that Browning gives it to us before we read a single line. We know from the title that we're in St. Praxed's Church. (Check out more on the title in "What's Up With the Title?") And we know from the note just after the title that we're in Rome, 15—.
Putting that all together, we know that our speaker, the bishop, is ordering his tomb at the Chiesa di Santa Prassede, in the year 15…something or other. Basically, we're in a Catholic church in Renaissance Rome during the sixteenth century.
Before we even hear one word fly out of the bishop's piehole, then, we already know quite a bit about what's going on in the world at this time. We know that we're in a period of great artistic achievement. As well, the Catholic church wields a lot of power in society, particularly in Italian society. (Just a century before, the Pope had moved the seat of the church from France back to Rome.) Finally, we know that the church's influence led to a great deal of corruption among its authority figures. (You can read more about that here.)
Given all of that, Browning's bishop is not so much a bad apple who just enjoys the look of lapis lazuli. He's representative of a historical moment, one in which humility and piety took a backseat to human greed and self-centeredness. You may think he's exaggerating a bit, but a quick tour of the church itself seems to confirm this idea. If it wasn't our bishop, somebody sure was into bling when they decked out the joint.