From D.A. Carson, on Paul's understanding of when cultural accommodation crosses the line into legalism.

"Paul refuses to circumcise Titus, even when it was demanded by many in the Jerusalem crowd,
not because it didn’t matter to them, but because it mattered so much
that if he acquiesced, he would have been giving the impression that
faith in Jesus is not enough for salvation: one has to become a Jew
first, before one can become a Christian. That would jeopardize the
exclusive sufficiency of Jesus.

To create a contemporary analogy: If I’m called to preach the gospel
among a lot of people who are cultural teetotallers, I’ll give up
alcohol for the sake of the gospel. But if they start saying, “You
cannot be a Christian and drink alcohol,” I’ll reply, “Pass the port”
or “I’ll think I’ll have a glass of Beaujolais with my meal.” Paul is
flexible and therefore prepared to circumcise Timothy when the
exclusive sufficiency of Christ is not at stake and when a little
cultural accommodation will advance the gospel; he is rigidly
inflexible and therefore refuses to circumcise Titus when people are
saying that Gentiles must be circumcised and become Jews to accept the
Jewish Messiah."

Taken from The Supremacy of Christ in a Postmodern World

 

HT: Justin Taylor