1 Corinthians 5

New King James Version (NKJV)

Immorality Defiles the Church  

 ¹ It is actually reported that there is sexual immorality among you, and such sexual immorality as is not even named^[a] among the Gentiles—that a man has his father’s wife! ² And you are puffed up, and have not rather mourned, that he who has done this deed might be taken away from among you. ³ For I indeed, as absent in body but present in spirit, have already judged (as though I were present) him who has so done this deed. ⁴ In the name of our Lord Jesus Christ, when you are gathered together, along with my spirit, with the power of our Lord Jesus Christ, ⁵ deliver such a one to Satan for the destruction of the flesh, that his spirit may be saved in the day of the Lord Jesus.^[b]
6 Your glorying is not good. Do you not know that a little leaven leavens the whole lump? ⁷ Therefore purge out the old leaven, that you may be a new lump, since you truly are unleavened. For indeed Christ, our Passover, was sacrificed for us.^{[c] 8} Therefore let us keep the feast, not with old leaven, nor with the leaven of malice and wickedness, but with the unleavened bread of sincerity and truth.

Immorality Must Be Judged
⁹ I wrote to you in my epistle not to keep company with sexually immoral people. ¹⁰ Yet I certainly did not mean with the sexually immoral people of this world, or with the covetous, or extortioners, or idolaters, since then you would need to go out of the world. ¹¹ But now I have written to you not to keep company with anyone named a brother, who is sexually immoral, or covetous, or an idolater, or a reviler, or a drunkard, or an extortioner—not even to eat with such a person.
¹² For what have I to do with judging those also who are outside? Do you not judge those who are inside? ¹³ But those who are outside God judges. Therefore “put away from yourselves the evil person.”^[d]

^{As God's children in the church, Paul makes it clear to me, in the above scripture, that we must judge each other, brothers and sisters in Christ, with regard to sexual immorality especially.  A close relative of mine told me recently how a former spiritual mentor became involved in a sexually immoral relationship.  This former mentor was confronted by my relative.  The person told my relative they had no right to judge what the person was doing.  My relative seemed very hurt by this prospect but did not back down.  Based on the above scripture, the actions of my relative show great humilty and honesty and true love for the person involved and great reverence for God too.  It is hard to tell those who we love, admire, and have come to understand in a deep way that they are wrong, but clearly Paul, one of our church fathers, writes that this is necessary.  If we Christians, children of God by the blood of Christ, don't hold each other accountable and show it by reproaching and distancing ourselves with those who say they "love God" but don't show it by their actions, we risk losing more than just a friend/mentor.   Based on the above scripture, we risk losing an entire community of believers who will be contaminated by the "leaven" of one person's willful disregard of God's law, which according to His Word never, ever changes.}