R.C. or not R.C.? That is the question.
Protestants who don't ask themselves why they're not Catholic (or Orthodox) puzzle me; as do Catholics (or Orthodox) who don't seek to convert Protestants to their respective communions.
I once listened to a Peter Gomes sermon, Where Are the Protestants?, hoping for some light on this important question. What I got was a pleasant sermon illustration on ecumenism (competing church bells of Harvard ought ring in harmony, not discord), along with Gomes' curious assertion that to be Protestant is actually about protest itself. Aside from the fact that this definition carries little historical weight, one wonders why adherence to a Protestant church is necessary to continue that good work of protest. In fact, if Gomes is right, then those who convert to Catholicism or Orthodoxy are the most genuinely Protestant, protesting as they are Protestantism's reduction to mere protest.
Gomes is to be commended, however, for at least attempting to publicly answer the question of the day, on which many Protestants observe an uncomfortable silence; uncomfortable because so many bright and faithful Protestants are Protestant no more.
It is natural for a Protestant to appeal to Scripture in tackling the question, as Scripture - not personal autonomy or protest - was, believe it or not, the point of the Reformation. Might there be one particular Bible passage with the power to keep someone Protestant? C.S. Lewis (whose reflection on this subject are most profound), suggested John 20:21-22 was appropriate. I'll add a few more possibilities, provided no one accuse me of "prooftexting" as a shortcut to thought. It this case the verses are an encapsulation of much (perhaps too much) thought.
1. 1 Cor. 7:20. Self explanatory, and by far the cleverest. Granted the "bloom where you're planted" does not account for toxic soil.If I may quote some earlier reflection, the competing claims of Catholics and Orthodox "should be more than enough to scramble any hasty conclusions." When struggling with this matter, as I often do, I think back to King Kong. Alone, the girl [Protestant ecclesiology] is no match for the beast, but then the T-Rex comes along.
2. Luke 11:27-28. In the 16th century Protestant arsenal, some verses were used illegitimately. This Lukan line, however, was fair game, and can still stir that Ulster blood. It's the Protestant equivalent to the Dyothelite's Matthew 26:39 (as to "Dyothelite," again I refer you to theo-blogging rule 3). Problem is, triumphantly pointing this verse as Protestant vindication requires a caricature of Catholic and Orthodox thought on Mary.
3. Along similar lines there's Mark 7:8, which still has some punch when protesting an undeniable historical tendency. Granted this requires ignoring John Henry Newman on development of doctrine.
4. Matt 8:20. Who says we get ecclesial satisfaction this side of things anyway? This one perhaps encapsulates Radner's thought, and gets problematic with extreme ecclesial dissatisfaction.
5. Gal. 5:6-8. This one's quite dangerous, as to make the direct Catholic/Orthodox parallel with the Judaizers would be to equate both those communions with "yokes of slavery" that "alienate from Christ." This is as unfounded as a Protestant claiming Catholics or Orthodox need become Protestant to be saved (though, as Peter Kreeft concedes here, on occasion they do). Still, the verse again warns of the danger in adding essentials to the gospel that aren't essentials.
6. Finally, while the Protestant with lingering anti-Catholicism needs consider Exodus 20:12, the Protestant overcome by desire to become Orthodox or Catholic might consider Exodus 20:17. Depending on your upbringing, that first one can work for staying Protestant as well.
Nevertheless, in the words of Timothy George, ours is a time when callings "to a deeper discipleship to Christ across the historic divides" are not uncommon. The point - it is so easy to forget - being deeper discipleship.