A phrase of Blake's which occurs in several contexts seems to carry more importance than is first apparent. 'Grain of sand' first appears in Milton in conjunction with the tiny fly with a brain open to 'heaven & hell'. Second Satan's search of 'every grain of sand' each night does not yield the Gate of Los which might allow entry into Eternity. Next it is the 'grain of sand' itself which Satan's Watch Fiends cannot find. We are warned against laying a finger in vengeance against any 'grain of sand'. In Auguries of Innocence we are introduced to the idea of seeing the world in a 'grain of sand'. Next we read that in Painting no 'grain of sand' is insignificant. In Blake's letter to Thomas Butts the 'grain of sand' is revealed as an instance of 'men seen afar'.
Milton, PLATE 20 , (E 114)
"Seest thou the little winged fly, smaller than a grain of sand?
It has a heart like thee; a brain open to heaven & hell,"
Jerusalem, PLATE 35 , (E 181)
"By Satans Watch-fiends tho' they search numbering every grain
Of sand on Earth every night, they never find this Gate.
It is the Gate of Los. Withoutside is the Mill, intricate, dreadful"
Jerusalem, PLATE 37 ,(E 183)
"There is a Grain of Sand in Lambeth that Satan cannot find
Nor can his Watch Fiends find it: tis translucent & has many Angles
But he who finds it will find Oothoons palace, for within
Opening into Beulah every angle is a lovely heaven
But should the Watch Fiends find it, they would call it Sin"
Jerusalem, PLATE 45 , (E 194)
"And he who takes vengeance alone is the criminal of Providence;
If I should dare to lay my finger on a grain of sand
In way of vengeance; I punish the already punishd: O whom
Should I pity if I pity not the sinner who is gone astray! "
Songs and Ballads, (E 490)
Auguries of Innocence
"To see a World in a Grain of Sand
And a Heaven in a Wild Flower
Hold Infinity in the palm of your hand
And Eternity in an hour"
Last Judgment, (E 560)
"Poetry admits not a
Letter that is Insignificant so Painting admits not a Grain of
Sand or a Blade of Grass much less an
Insignificant Blur or Mark"
Letters, 16, (E 712)
[To] Mr [Thomas] Butts, Great Marlborough Street
Felpham Octr 2d 1800
"For they beckond to me
Remote by the Sea
Saying. Each grain of Sand
Every Stone on the Land
Each rock & each hill
Each fountain & rill
Each herb & each tree
Mountain hill Earth & Sea
Cloud Meteor & Star
Are Men Seen Afar"
Christ on the Pinnacle of the Temple
There is a sense in which God allows himself to be distributed in man as Christ. Blake seems to be using 'grain of sand' as an image of the internalized Christ who exists within the soul of each individual. Through the 'grain of sand' man sees his own true nature as a child of God and enters Eternity.
'Grain of sand' is apropos as an image of a distributed or scattered form of existence as is the bread of communion which is offered as the broken body which is to be eaten.
 And when he had given thanks, he brake it, and said, Take, eat: this is my body, which is broken for you: this do in remembrance of me
 The cup of blessing which we bless, is it not the communion of the blood of Christ? The bread which we break, is it not the communion of the body of Christ?
 Now ye are the body of Christ, and members in particular.
In Greek mythology we learn of a similar usage of a spark of existence which was distributed through the breaking of a body. Edward F. Edinger writes in The Eternal Drama: The Inner Meaning of Greek Mythology about the myth of Dionysus:
"I am of your blessed race" refers to the myth of the dismemberment of the infant Dionysus by the Titans. It will be recalled that the Titans ate Dionysus except for his heart, and Zeus then destroyed them with a thunderbolt, but of the ashes man was made, and man thus contains a remnant of the divine spark of Dionysis. The soul declares that he has the Dionysian spark in him because he is made of Titan dust." (Page 166)
Blake's phrase 'grain of sand' symbolizes the individual's internal opening for the entry of the spirit which we all share but which we each experience as uniquely our own.