Since you feel “strongly led to simply pose a question…”, I wonder if it isn’t best to just wait upon the Holy Spirit to lead you to the proper question when the time comes.
Many fine questions have been suggested, and I can certainly add my own to the mix but I think that much depends on 1) the particular tradition and views of the group and 2) the particular reason you have been invited to participate.
On the one hand, if you are invited because they want to “win you back” from us horrible Catholics…they may not really be interested in any question you ask…their minds will be closed. In such a case you will likely have little opportunity to talk - let alone communicate.
On the other hand, if they are more open minded they might be truly interested in the whys of your journey and you may find yourself the center of conversation. They will be full of questions about your particular journey and what has led you to where you are. These answers only you can know.
All that said, for me, the most telling aspect for me is the very real and consistent call to unity that we see in the NT. Here are a few…
20 "I do not pray for these only, but also for those who believe in me through their word, 21 that they may all be one; even as thou, Father, art in me, and I in thee, that they also may be in us, so that the world may believe that thou hast sent me.
5 May the God of endurance and encouragement grant you to live in such harmony with one another, in accord with Christ Jesus, 6 that together you may with one voice glorify the God and Father of our Lord Jesus Christ.
1 Cor 1:10
I appeal to you, brethren, by the name of our Lord Jesus Christ, that all of you agree and that there be no dissensions among you, but that you be united in the same mind and the same judgment.
2 Cor 13:11
Finally, brothers, rejoice. Aim for restoration, comfort one another, agree with one another, live in peace; and the God of love and peace will be with you.
Only let your manner of life be worthy of the gospel of Christ, so that whether I come and see you or am absent, I may hear of you that you are standing firm in one spirit, with one mind striving side by side for the faith of the gospel,
1.I therefore, a prisoner for the Lord, beg you to lead a life worthy of the calling to which you have been called, 2. with all lowliness and meekness, with patience, forbearing one another in love, 3. eager to maintain the unity of the Spirit in the bond of peace. 4. There is one body and one Spirit, just as you were called to the one hope that belongs to your call, 5. one Lord, one faith, one baptism, 6. one God and Father of us all, who is above all and through all and in all.
1 Pet 3:8
Finally, all of you, have unity of mind, sympathy, brotherly love, a tender heart, and a humble mind.
complete my joy by being of the same mind, having the same love, being in full accord and of one mind.
All of these call for a deep and profound unity…not a loose, “agree to disagree”, “invisible church” unity.
Then there is Jesus instruction for settling issues - “tell it to the Church and to listen to the Church” (Mt 18:15-18) and the example of that instruction that we find in Acts 15…the Church acting universally and in council to settle a doctrinal matter.
I have - on a number of occasions - shown this evidence to protestants and then asked them to show equally compelling biblical evidence for the “protestant” church model (locally independent etc). I am almost always met with a profound silence.
It might be interesting to see how your protestant group would react to this weight biblical of evidence for a visible and authoritative Church.
But that is just me…
you really need to go with what you are most comfortable with…and what the Spirit leads you to…