The first most impressive symbol in Le Morte d'Arthur by Malory may be the sword Excalibur. In the story, whoever wields the sword is the recognized King of England. Because of its importance, the sword symbolizes power. (It is also said to symbolize the responsibility that comes with power, and may allude to Arthur's eventual fall, when he loses sight of his responsibility as King.)
The Round Table is also symbolic of equality. Because it is round, no one sits at the "head" of the table, and therefore, no one is more important that his neighbor.
The Round Table was first mentioned...in 1155 and in that account was made round so that all the knights seated around it would have the same stature...
Christian themes are strongly present in the story. Christ is alluded to when the young maiden walks across the water of the lake to retrieve Excalibur for Arthur in one version of how Arthur becomes king.
Here we see the commingling of Christian and pagan motifs, the Christ-like walking on water with the fairy quality of Excalibur and the mythic making of a King.
This is a direct parallel to Christ's miracle of walking on the water, as recounted in the Bible in the books of Matthew, Mark and John. This act by the maiden may symbolize God's desire that Arthur be King. (The English believed that whoever was King or Queen was ordained to be such by God. Anyone killing a king was believed to have committed a mortal sin, as is seen with the ill-fated Macbeth after killing Duncan in Shakespeare's Macbeth, as well as Claudius after killing Old Hamlet in Shakespeare's Hamlet.)
The partaking of the meal around the Round Table has religious overtones as Arthur instructs that an empty seat be left to remember Judas Iscariot, the disciple who betrayed Christ after the Last Supper. In fact, the image of the knights as they gather around the table seems to evoke a mental picture of the Last Supper. Its symbolism here may allude to the noble brotherhood of the knights and their calling to protect each other, joined by their service of God and King.
The pursuit of the Holy Grail is also associated with Christianity. It is...
...most often identified with the dish, plate, or cup used by Jesus at the Last Supper...
The search for the Grail is something that is carried out by several of Arthur's knights—but only "the finest knight...will be guided by God..." to find the Grail.
...Joseph of Arimathea [is closely connected] with the Grail legend [which says that] Joseph receives the Grail from an apparition of Jesus...
According the legend, the Grail is sent by Joseph with his followers to Great Britain, so it would make sense that the knights would seek it in England.
The search for the Grail may be seen as symbolic to the knights' search for goodness and even the blessings of God, to whom they owed allegiance—even beyond that which they owed their King.