Now you, brothers, like Isaac, are children of promise.
In these verses the apostle illustrates the difference between believers who rested in Christ only and those judaizers who trusted in the law, by a comparison taken from the story of Isaac and Ishmael. This he introduces in such a manner as was proper to strike and impress their minds, and to convince them of their great weakness in departing from the truth, and suffering...
Commenting on Galatians 4:21-31
But as then,.... In the times of Abraham, when Hagar and Sarah, the types of the two dispensations of the covenant, and Ishmael and Isaac, the figures of the two different seeds, the natural and spiritual seed of Abraham, legalists and true believers, were living: he that was born after the flesh; which was Ishmael, who was a type, or an allegorical representation of such...
Verse 28. Now we, brethren. We who are Christians. Are the children of promise. We so far resemble Isaac, that there are great and precious promises made to use. We are not in the condition of Ishmael, to who no promise was made. (c) "we, brethren"