Over the years a church in the area has had special Christmas and Easter concerts, for which they hire musicians through the union. There seems to be no pretense about this, or effort to conceal that these folks are not church members. Part of my concern is that the primary aim seems to be an impressive performance -- why use lesser talent that comes from within the church? (Of course they will use top performers who are in the church.) My main concern is 2 Corinthians 6:14. Why should we be unequally yoked with unbelievers in praising God and proclaiming the gospel?
Using the logic you have described one might hire unbelievers for staff positions (choir director?) because they might hear the gospel and be saved.
In 1 Corinthians 7:15-16 Paul tells the believing partner to let the unbeliever go (rather than to try to force them to stay in the marriage), and asks (in effect), "Do you really think that forcing your unbelieving husband to stay in the marriage will save him?" So why do we really think that the only way to evangelize unsaved musicians is by hiring them to come to church to perform?
By the way, I have no problem with hiring an unbelieving plumber to fix a leaking pipe. He is not engaged in leading people in worship.
There are several other texts that might apply here:
18 And do not get drunk with wine, which is debauchery, but be filled by the Spirit, 19 speaking to one another in psalms, hymns, and spiritual songs, singing and making music in your hearts to the Lord, 20 always giving thanks to God the Father for each other in the name of our Lord Jesus Christ (Ephesians 5:18-20).
16 Let the word of Christ dwell in you richly, teaching and exhorting one another with all wisdom, singing psalms, hymns, and spiritual songs, all with grace in your hearts to God. 17 And whatever you do in word or deed, do it all in the name of the Lord Jesus, giving thanks to God the Father through him (Colossians 3:16-17).
In these two texts music is viewed as part of the instructive process.
Thus, it would seem, to hire unsaved musicians to perform in a church service is to hire them to participate in spiritual instruction. That, to me, is problematic.
1 When any of you has a legal dispute with another, does he dare go to court before the unrighteous rather than before the saints? 2 Or do you not know that the saints will judge the world? And if the world is to be judged by you, are you not competent to settle trivial suits? 3 Do you not know that we will judge angels? Why not ordinary matters! 4 So if you have ordinary lawsuits, do you appoint as judges those who have no standing in the church?
5 I say this to your shame! Is there no one among you wise enough to settle disputes between fellow Christians? 6 Instead, does a Christian sue a Christian, and do this before unbelievers? (1 Corinthians 6:1-6)
Here in 1 Corinthians Paul is rebuking the Corinthian saints for taking their disputes to the courts, where unbelievers will be judges in matters that are, in essence (at their root), spiritual. Paul asks, "Don't you have someone in the church who can do this?" Why do we take our musical programs and seek out secular performers. Are there not saints who can do this? I find it very difficult to hear the Handel's Messiah performed in church by many who are spiritual dead.