‘Looking carefully lest there be any man who falls short of the grace of God; lest any root of bitterness springing up trouble you, and thereby the many be defiled.'
And this seeking of peace and sanctification should be carried through with greatest care as they keep their eyes open to ensure that none of them ‘fall short of the grace of God'. For those who are in the grace of God (God's action towards us in unmerited love and favour) it is impossible to fall short of it, for it is God's gift whereby we are His workmanship and whereby He will make us truly righteous in deed as well as in standing (Ephesians 2:9-10). The idea here is rather of someone who falls short of God's grace that has been offered to them, by a refusing to believe in Him truly in genuine response, by a holding out on His calling. They will be revealing that they have not yet truly become His, and such persons should be the concern of all God's people.
‘Lest any root of bitterness springing up trouble you, and thereby the many be defiled.' For he is concerned lest there be those among them who have within them a bitterness at what they are facing which like a root will spring up and spread and begin to produce a more mature fruit of bitterness, causing many to be led astray (compare for the language Deuteronomy 29:19 where the idea is used of turning from God to false religion). They may feel that they had followed the Messiah expecting him to lead them into pleasant paths, and that He had clearly failed because of their present situation. And once such ideas begin to be mooted they can soon spread, and he is fearful lest it weaken the church in its faith and in its resolve.
‘Thereby the many be defiled.' Being defiled is the opposite of being made holy. They cease from their separation to God and become worldly minded because their faith has dwindled. This may then manifest itself either in sexual misbehaviour, or in being taken up with the world so that heavenly things cease to be important and their ‘holiness', their outward separation to God, is marred.