The speaker of this poem directly addresses her beloved, trying to explain the depth of her feelings for that loved one. She says,
I love thee with the passion put to use
In my old griefs, and with my childhood's faith.
In other words, in the past, the speaker felt overwhelming grief, a grief that seemed to consume her whole self. She apparently grieved passionately and fully. However, now she loves her beloved with that same amount of passion; her grief seems to be gone, and it has been replaced with an equally as passionate love for another. She also claims that she loves this person with as much certainty as she used to feel faithful as a child. Children tend to accept rather than question or reject, as a result of their innocence and lack of experience. The speaker says that, rather than retaining that same faithfulness that she possessed as a child, she now feels that same sense of certainty in her love and lover.
Further, the speaker says,
I love thee with a love I seemed to lose
With my lost saints.
When we are children, we often feel intense admiration and love for people who seem like heroes to us; however, sometimes aging brings with it more knowledge and less faith in those people. We realize that those we thought were perfect, like saints, are actually human and fallible. The speaker used to love like this but seems to have been disillusioned with her one-time "saints"; now, though, she loves her beloved with that same kind and strength of love.