When Paul says, let us keep the feast (vs. 8), he is referring to the Feast of Passover and Unleavened Bread. The New Testament church kept the Feasts of the Lord! Keeping them teaches, in a tangible visual aid way, to a society and a people who did not have books, the history of Israel; the gospel and plan of salvation; the nature and ministry of the Messiah; prophecy; and theological and spiritual principles, in a way that even five- year- olds could understand.
So now, some might ask, that we all have Bibles, can we ignore the Feasts? Well, leaving out the fact that in this chapter Paul specifically told the church to keep the Feast with sincerity, I have not found that position anywhere in Scripture, Old or New. I am not smarter than God, so I would not presume to make that leap. In fact, Paul admonishes the Gentile church to imitate him as he imitates Messiah (1 Cor 11:1), and both Paul and Messiah kept the Feasts and all Torah.
On a bit different track, some parents use this Scripture:
“I wrote to you in my epistle not to keep company with sexually immoral people. 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.” 1 Cor 5:9-10
As an injunction to send their children to public school, so that their children can be salt and light in the public schools. I know a lot of parents do that – 90%, I read in one Barna research survey, of the children of evangelical parents are educated in the public schools. But I believe Paul is addressing his admonition to adults, not children. In Scripture, the unit of child rearing and teaching is the home, under the direction of mothers and fathers. Children are weak, spiritually, and are foolish by reason of their age and immaturity. You do not put the weak or the foolish in with the ungodly, because the weak follow, they do not lead. This principle is found throughout Proverbs and the Scripture!