We boast in many things, the majority of which tends to be sinful. Paul indicts the Corinthian church of boasting in immorality (e.g., the son committing adultery with his mother) because they did not discipline this immoral act. This is perhaps grounded in their perverted view of what it means to have “freedom in Christ” (1 Cor.6:12-20). The point here is their boasting is sinful because it glorifies sin and Paul uses the metaphor of leaven to explain it:
“6 Your boasting is not good. Do you not know that a little leaven leavens the whole lump of dough? 7 Clean out the old leaven so that you may be a new lump, just as you are in fact unleavened. For Christ our Passover also has been sacrificed.”
The apostle here reminds the Gentile Corinthian church of their roots in the Passover meal which Christ came to personify and fulfill through his death as the Passover Lamb (his body represented in the unleavened bread) holy and pure.
In the beginning of this letter, Paul describes the Corinthians as the “called” and as “saints” even though their lives were imbibing the world’s “wisdom” and its darkness. The apostle (as God’s divinely appointed spokesman) is commanding zero tolerance for compromise to the Church because like a virus it will spread and eventually destroy the whole body (e.g., leaven, lump, dough). Moreover, just as Christ is the Passover Lamb who died to sin and is now alive to God, so to the Corinthian’s are to emulate the Master in their sexuality (E.g., Rom.6:1-14), not the wisdom of this world with its’ “enlightened” and “liberated” views of sexual expression that is often praised among the unregenerate. Paul continues:
“8 Therefore let us celebrate the feast, not with old leaven, nor with the leaven of malice and wickedness, but with the unleavened bread of sincerity and truth.”
So, since we belong to Christ and are in this world that’s “leavened-sinful”, we’re to celebrate that feast (i.e., rejoice in God’s holiness that believers partake in) and not go back to “Egypt” into the slavery of the world with it’s greed, malice, wickedness, immorality, etc. We are to ground our actions in what is sincere (i.e., un-hypocritical) and in the truth (i.e., what’s objectively true—Christ our Passover Lamb, risen from the grave).
The relevance of this passage can’t be overstated. Adultery, fornication, and all kinds of sexual expression contra God’s design for human flourishing, not human misery, as some contend, are leaving image bearers empty, confused, unfulfilled, and eventually if un-repented of, will take them into a Christ-less eternity (i.e., Hell).
When believers buy into the prevailing “Same-sex” marriage and “Transgender” rhetoric of legitimizing its’ position which is blatantly contra design, are we not drinking in the “wisdom” of this world? Yes, we are and far from being an expression of love, it’s an expression of treason against the self-existent Creator, who alone is the ground of what is beautiful, good and true, not the finite, feeble, dependent creature.