It is not a judgement on who is and is not a Christian. What was at stake during the Reformation and what is at stake now is whether people believe the true Gospel or the false gospels. The question is very simple: what is the true Gospel of Jesus Christ and do you believe it? Both the reformers like Luther, Calvin, and Chemnitz and the bishops of the Council of Trent, like Paul III, Psaume, and Pious IV understood the stakes:

"I am astonished that you are so quickly deserting him who called you in the grace of Christ and are turning to a different gospel--
not that there is another one, but there are some who trouble you and want to distort the gospel of Christ.
But even if we or an angel from heaven should preach to you a gospel contrary to the one we preached to you, let him be accursed.
As we have said before, so now I say again: If anyone is preaching to you a gospel contrary to the one you received, let him be accursed. (Galatians 1:6-9)

"For it is time for judgment to begin at the household of God; and if it begins with us, what will be the outcome for those who do not obey the gospel of God?
And 'If the righteous is scarcely saved, what will become of the ungodly and the sinner?'" (1 Peter 4:17-18)

If one teaches or believes another gospel than the one explicitly taught by Scriptures, they remain dead and condemned in sin and will go to eternal damnation after death.

The Council of Trent codified ex cathedra that anyone who teaches that grace is only the flavor, that is the work, of God, he is damned (Canon XI).

While the Reformers promoted the view being espoused by Shalltear-stan. Rome and Wittenberg cannot both be right.