I cannot love thee as I ought, For love reflects the thing beloved; My words are only words, and moved Upon the topmost froth of thought.
"Yet blame not thou thy plaintive song," The Spirit of true love replied; "Thou canst not move me from thy side, Nor human frailty do me wrong.
"What keeps a spirit wholly true To that ideal which he bears? What record? not the sinless years That breathed beneath the Syrian blue:
"So fret not, like an idle girl, That life is dash'd with flecks of sin. Abide: thy wealth is gather'd in, When Time hath sunder'd shell from pearl."
This first stanza seems to suggest that the speaker is unable to love Arthur (now the spirit-Arthur) as he should.
"[L]ove reflects the thing beloved," and since Arthur is so much higher than Tennyson now (literally and figuratively), his (Tennyson's) words can only reflect the surface.
This is nicely presented as a thought floating on the top of ocean waves, which is suggested by "topmost froth" in the first stanza.
Love kind of scolds Tennyson for this thought, and tells him that he won't get rid of her that easily, silly mortal man.
She tells him to not worry like a little girl that he has sinned during his life. At the end of time, his worth will be revealed like a pearl, and his sins will be like the oyster shell that has been worn away.
So, maybe "the Spirit of true love" in the second stanza is Jesus and not the spirit of secular love. Or maybe Tennyson is working on both levels.