There are any number of interesting aspects to the painting which I would point out, the first of which being the catechetical dimension of the painting. Evidently the context around the work is that it is set within a church situated within the Spanish colonial missions. As such, imagery that would help to expound upon Catholic doctrine makes manifest sense for it is a powerful means by which to impart and reinforce these teachings in new lands and to new peoples. So what does this painting intend to teach? Well, for one, the fact of purgatory itself and, for another, the power of the Sacrifice of the Mass to assist the Holy Souls who find themselves there. This aspect is emphasized by the presence of Pope St. Gregory the Great, father of sorts to the Roman Mass, and of course the image of the Requiem Mass itself with souls being released from purgatory seen in behind.
In the second half of this painting we see the souls in purgatory -- with popes, cardinals and bishops featuring prominently. While this may well have intended to teach that even great churchmen are sinners, another possible explanation is that their inclusion intends to serve as a reminder that all men die; that all are equal before God.
In addition to this catechetical aspect, another point of interest to be found here is liturgical in nature. Here again we remember the context -- the Spanish missions -- and we can see this quite clearly reflected in the vestments which are of the Spanish cut, complete with the Spanish 'collarin.' What's more, being a Requiem Mass, we see two silver (not gold) candlesticks on the altar; an altar which is covered by an antependium and set on top of a carpeted predella.
Why this is of interest, at least to me, is that it reflects what would have been liturgically taking place in the Spanish missions. It is a testament to the evident priority that was given to the sacred liturgy and to the care which was taken to bring and to pass on this noble liturgical tradition in all its fullness and richness.
