Read 1 Corinthians 5 at Bible Gateway.
This chapter forms a chiastic structure:
1 Cor 5
1a) 1 Cor 5:1-2, Sexual immorality present and tolerated in the church;
1b) 1 Cor 5:3-5, Paul has judged him who has done this deed;
1c) 1 Cor 5:6-7a, A little leaven leavens the whole lump, therefore purge out the old leaven;
1d) 1 Cor 5:7b, That you may be a new lump since you truly are unleavened;
central axis) 1 Cor 5:7c, For indeed Christ, our Passover, was sacrificed for us;
2d) 1 Cor 5:8, Keep the feast with the unleavened bread of sincerity and truth;
2c) 1 Cor 5:9-11, Do not keep company with anyone named a brother who does these things;
2b) 1 Cor 5:12-13a, We do not judge those who are outside, but inside the church;
2a) 1 Cor 5:13b, Put away from yourselves the evil person.
Where did Paul get the idea that a man having a relationship with his father’s wife was immoral? From Torah (Lev 18, especially vs. 8). Paul calls a person who names himself a brother, inside the church, who practices sexual immorality as defined by Torah, an evil person (the a pair). Paul considers Torah as God’s definition of righteousness and sin, good and evil, revealed to man.
The argument is, why should it matter what two consenting adults do in the privacy of their own home? No one is being hurt. 1 Cor 5 is Paul’s answer to that argument. He bases his definition of right and wrong, good and evil, on God’s revealed word, and chastises the church for being boastful in themselves when they are allowing this kind of thing to go on, instead of mourning in repentance for the sin in their midst.
I read a discussion last week, which shocked me. A popular Christian marriage and family speaker declined to continue to support Starbucks because they have come out in their official company policy in support of same sex marriage. He is a marriage speaker; his calling is to help couples build their marriages on solid and biblical foundations, for their happiness. The amount of feedback he received from his Christian supporters on how wrong it was for Christians to oppose a loving relationship based merely on gender was overwhelming! I was in shock.
Based on 1 Cor 5, this marriage and family speaker was following Paul’s example for the church, and those Christians who thought him narrow- minded would be at odds with Paul for acceptable standards within the church. Many people say today that the Bible’s opposition to sexual immorality as defined in Torah is Old Testament — we can throw the Old Testament out and simply follow the New Testament, which is all based on love without conditions or standards. That is NOT a true statement.
In fact, only the God who is Love can define what is and is not Love for fallen man. Man’s definition, where it conflicts with God’s, has selfishness (promoting what is good for me but harmful for you) at its core.
The entire New Testament is built upon the foundation of the Old Testament, especially Torah, and there is not a single word in Torah that can be excised without causing the entire New Testament to come crashing down. It is a lie that Jesus and Paul and the other apostles swept away Torah standards and obedience for the church, and this is one of the places in the New Testament that proves it.
Update: I just want to clarify something that might have been misunderstood. The evil person is not the one who practices sexual immorality as defined by Torah, who is out in the world. How can they be? They are out in the world! The one Paul terms the evil person, is the one who is inside the church, who claims to be a brother, who practices sexual immorality as defined by Torah. Why? Because they claim to have been transformed by the power of the shed blood of Christ our Passover who has been sacrificed for us (central axis)!