Galatians 2:17 is the five finger exploding heart death punch of Galatians 2:

"But if, in our endeavor to be justified in Christ, we too were found to be sinners, is Christ then a servant of sin? Certainly not!"

As I was studying once for a sermon on Galatians 2:15-21, this verse gave me the most fits. It looks straightforward enough, but it is deceptively complex.

First, I am seeing Paul turning the language of the legalistic Judaizers on its head. There is an echo here of "Shall we sin all the more so that grace may abound?" The more I gnawed on Gal. 2:17 the more clearly I could see that Paul is sort of saying "If justification is not by faith alone in Christ alone, should we circumcise all the more so that justification may abound?" (And in Galatians 5:12 he does kind of say that.)

In a nutshell Paul is saying "If justification in Christ alone reckons us still 'sinners,' Christ's work is worthless and he is a minister of sin."

But something else catches my eye. Still playing off the Judaizing zeal, he uses the phrase "endeavor to be justified in Christ." Why would he use the word "endeavor"? Isn't the truth he's proclaiming about the end of endeavoring to be justified? That justification comes via faith (not works) in the finished endeavor of Christ?

Yes, but there's something else here besides just tweaking the legalistic "work" lingo. There is a very real sense in which we must endeavor to be justified in Christ. This idea is repeated in Hebrews 4:9-11:

So then, there remains a Sabbath rest for the people of God, for whoever has entered God’s rest has also rested from his works as God did from his.
Let us therefore strive to enter that rest, so that no one may fall by the same sort of disobedience.

Strive to enter rest? Endeavor to stop endeavoring? Labor to stop laboring. Why this language?

The gospel bids us strive to stop striving.

Because it takes conscious and concerted effort to orient our stubborn selves around the gospel. Our flesh yearns for works, for the merits of self-righteousness, so it's hard work to make ourselves rest in the finished work of Christ. It is a daily work, the labor of crucifying the flesh, taking up the cross, and faithfully following he who has finished the labor.

"Even after you are converted by the gospel, your heart will go back to operating on other principles unless you deliberately, repeatedly set it to gospel-mode."
-- Tim Keller

"Most necessary it is, therefore, that we should know the gospel well, teach it unto others, and beat it into their heads continually."
-- Martin Luther