The Galatians were being tested but they had not completely abandoned the Truth.  They were at a crossroads and had to decide which way they should go.  In order to make the right choice they had to listen to the right voice.  Paul was trying to be the voice of reason for them and helping them consider the consequences of their choices before they made a decision on which path to follow.  

I.   You have to be well informed to choose the right path. vs 21 

II.  History helps us understand which path to choose. vs 22-23

III. Understand the covenant promise helps us choose the right path. vs 24-27

IV. Making an accurate application helps us choose the right path.