The ace of cups has some fascinating imagery from a Christian perspective. This illustration comes from the Illuminated Tarot, which in turn is derived from the Rider Waite Tarot.
You will notice that the chalice has streams of living water flowing from it. That a white dove, representing the Holy Spirit, is decending upon it with a communion wafer, inscribed with a cross, in its beak. The cup in turn is held by the hand of God, eminating from the clouds.
This wealth of communion imagery reveals this cup is the grail, the cup of the last supper of Jesus of Nazareth.