Galatians 5:9 - Matthew Poole's English Annotations on the Holy Bible

This is a proverbial expression, which (as others of that nature) is applicable in more cases than one. The apostle made use of it, 1 Corinthians 5:6, to persuade that church to purge their communion, by casting out the incestuous person: he maketh use of it here to persuade them to take heed of admitting any principles of false doctrine, which he compareth to leaven, (as our Saviour does, Matthew 16:6,12), and that very fitly, both in regard of the sour and of the diffusive nature of it; the latter of which is here chiefly intended; the truths of God having such dependence one upon another, that he who erreth in any one doctrine of faith, seldom continueth long sound as to other points.