The beauty is described specifically as "sweet," and I think that innocent message carries through the poem. It is also worth noting that the poem has a definite Christian undercurrent, so perhaps the message of Christ is the true "beauty" the speaker admires. In line 11, the speaker brings the words "praises" and "prophecies" together in a singular line to underscore this connection. All things considered beautiful, such as "lovely knights," pale in comparison to the beauty of Christ (or this specifically unnamed man). The sonnet is reminiscent of Psalm 40:5:

Many, Lord my God,
are the wonders you have done,
the things you planned for us.
None can compare with you;
were I to speak and tell of your deeds,
they would be too many to declare.

James 3:9 says

With the tongue we praise our Lord and Father . . .

This line is similar to the final conclusions of the speaker as he notes that he does not have the capacity to praise the unknown man of the poem; however, the speaker still has wonder-filled eyes at the indescribable beauty of this Truth he beholds.

The flawless beauty of Christ could be well-supported as the true subject of the poem, and the speaker then overflows in adoration.