The entire poem is spent praising all of God's beautiful creations. The poem opens to God, saying, "Glory be to God", and then ends with a similar note, saying, "Praise Him." The poem, with its list of colors, animals, and nature is a list of beautiful and wonderful things that have been created by God. It revels in the "skies of couple-colour as a brinded cow", the "Landscape plotted and pieced—fold, fallow, and plough" and even things that are "fickle" or changeable. Then in line ten, it refers to God, "whose beauty is pást change". It is referring to God there; the poem describes nature's beauty that fades and dies, but God's beauty is always constant, and always the same. He is unchangeable. The world is, but God is not.
I provided a link to a great line-by-line summary that will also help. Good luck!
Also, the number of strong beats in each line is constant--Hopkins invents a metrical form called "sprung meter": though the number of syllables may change from line to line, the number of stressed beats in each line stays the same. Thus, the metrical form, too, supports cadena's wonderful answer about the poem's glorification of a constant God in an inconstant world.