Paul rebukes the Galatians for their return to legalism

Gal. 4:8-11:

However at that time, when you did not know God, you were slaves to those which by nature are no gods. 9 But now that you have come to know God, or rather to be known by God, how is it that you turn back again to the weak and worthless elemental things, to which you desire to be enslaved all over again? 10 You observe days and months and seasons and years. 11 I fear for you, that perhaps I have labored over you in vain.