The speaker begins by describing the scene. It is a calm and beautiful evening, and the sun is setting peacefully as the sky hangs over the sea:
It is a beauteous evening, calm and free,
The holy time is quiet as a Nun
Breathless with adoration; the broad sun
Is sinking down in its tranquillity;
The gentleness of heaven broods o'er the Sea:
At line six the speaker begins to address someone who turns out to be a young girl. He tells her to listen, that "the mighty Being is awake" and making a "sound like thunder" that lasts forever:
Listen! the mighty Being is awake,
And doth with his eternal motion make
A sound like thunder--everlastingly.
The speaker then tells the child (actually his daughter, Caroline) who is walking beside him that even though she isn't affected by the solemn ideas he has when he comes face to face with nature, she is not any less divine. In fact, she "liest in Abraham's bosom all year," because God is with her even when she is not aware of Him:
Dear Child! dear Girl! that walkest with me here,
If thou appear untouched by solemn thought,
Thy nature is not therefore less divine:
Thou liest in Abraham's bosom all the year;
And worship'st at the Temple's inner shrine,
God being with thee when we know it not.
"It is a beauteous evening" does exactly what its title implies it will--it describes a beautiful evening scene--and yet this sonnet goes far beyond aesthetic pleasures, paralleling a simple walk along the beach with the religious power that Wordsworth feels in nature.
The poem gains even more power when the reader learns that the child Wordsworth walks with is his daughter Caroline, whom he has not seen in ten years because he has been separated from her and her mother by the war in France. The child's innocence is inspirational: even though she is not actively considering the power of the nature that surrounds them, she is a part of it nevertheless. And because, for Wordsworth, the very fact of being in nature is enough to inspire a powerful religious experience, he envisions his pure daughter standing alongside God, as if she has been accepted into heaven well before the hour of her death.