In Alfred, Lord Tennyson's poem Idylls of the King, what are some of the main themes in the section VIII -- the section about the holy grail?

In Alfred, Lord Tennyson’s poem Idylls of the
King, the section dealing with the holy grail deals with a number of
important themes.  Some of these are suggested early in this segment of the
Idylls, when Percivale, a one-time knight of King Arthur’s
roundtable (although now a religious hermit) is asked by an elderly priest (Ambrosius)
why he left the round table:

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‘Tell me, what drove thee from the Table
Round,
My brother? was it earthly passion
crost?'

'Nay,' said the knight; 'for no such passion
mine.
But the sweet vision of the Holy Grail
Drove me from all
vainglories, rivalries,
And earthly heats that spring and sparkle
out
Among us in the jousts, while women watch
Who wins, who falls;
and waste the spiritual strength
Within us, better offered up to
Heaven.'

In this response,
Percivale suggests many of the themes of the section as a whole.  These include the
following:

• the need to control earthly passions
  if one hopes to be a worthy follower of Christ and if one hopes to lead a worthy earthly
  life.


• the need to fix one’s attention on Christ
  (represented by the Holy Grail) if one hopes to be one of Christ’s worthy followers and
  if one hopes to lead a life truly worth living.


• the need
  to fix one’s attention on lofty goals and elevated purposes if one hopes to achieve a
  spiritually satisfying existence that will also be useful to
  others.


• the need to make sure that one’s ambitions and
  goals are truly elevated and worthwhile and that they are not rooted in selfishness and
  pride, the root of all sin.


• the need for true humility if
  one hopes to accomplish anything of genuine value and to be worthy of
  Christ.


• the real difficulty of attaining the spiritual
  and practical goals already enumerated.


• the alluring
  attractiveness of earthly temptations, which can distract one from goals that are truly
  worthwhile.


• the need to resist sexual temptations in
  particular, or at least the temptation to want to “show off” before members of the
  opposite sex.


• the need to employ wisely the spiritual
  potential that God grants to all human beings and to use this gift to the glory of
  God.


• the need to focus, ultimately, on the afterlife
  rather than on mere earthly existence. Everything we do on earth should be motivated by
  sublime, lofty ideals.

Elsewhere in “The Holy
Grail” section of Idylls (particularly near the end), the point is
made that not all human beings can attain the kind of spiritual purity attained by
Galahad. Yet one can still live a life useful to others if one focuses on more limited
but still valuable goals.