This moving sonnet starts off initially as a reflection on the strange way that time has of quickly passing us by. The speaker reflects how time, whom he personifies as "the subtle thief of youth," has "stolen" his "twenty-third year." However, this leads the speaker to reflect about not just his maturity in terms of his age, but also the maturity in terms of his gifts. Thus the speaker refers to his "inward ripeness," that "timely-happy spirits endu'th." This is a clear reference to the poetic gifts that the speaker feels he has been given, though they may not have fully matured like he has in his physical body. However, above all, he entrusts his life and his poetic giftings to God, who is the source of both. Note how the sonnet ends:
All is: if I have grace to use it so,
As ever in my great Task-Master's eye.
Thus we can say that the theme of this poem is entrusting our talents and gifts into God's hands, trusting that he will develop and mature them, as well as giving us the correct time and opportunity to practise, develop and use them for His glory.