Today's Gospel reading is on the importance of obedience to God's Will and doing good instead of being tied down with the observance of the Jewish religious institution of the Sabbath.
Jesus manifested this new way of observing the Sabbath so as to be free from the clutches of legalism and to respond to the needy. Refusal to do good is tantamount to sin, as found in James 4:17 or the sin of omission in today's understanding.
The first reading illustrates the sin of immorality and how the community responded to such a situation. 1 Cor 5:5 talks about expelling the one who committed the offence. This may mean that the person is cut off from the community. The rationale is that being ostracised from the Church will cause the person such anguish that he will repent and forsake his wrong doing. This may not work in today's reality!
The other image is the comparison of little yeast to the whole batch of dough. This is used to illustrate Christian holiness and discipline. The prohibition against yeast in the bread eaten during the Passover feast symbolised evil or sin. The Church here is called to get rid of the yeast of sin, wickedness and corruption.
The feast of unleavened bread symbolises living the Christian life in holy dedication to God and not getting involved in such sins as malice, wickedness and incestuous marriages.
Lord, grant us the grace to be conscious of the temptations which draw us away from God to do good for others.