John Calvin Commentary Romans 10:2

"For I bear them witness that they have a zeal for God, but not according to knowledge." — Romans 10:2 (ASV)

For I bear to them a testimony, etc. This was intended to lend credence to his love. There was indeed a just reason why he should regard them with compassion rather than hatred, since he perceived that they had fallen only through ignorance, and not through malice, and especially as he saw that they were led only by some regard for God to persecute the kingdom of Christ.

Let us learn from this where our good intentions may lead us, if we yield to them. It is commonly thought a good and very suitable excuse when someone who is reproved pretends that they meant no harm. And this pretext is considered valid by many today, so that they do not apply their minds to find out the truth of God, because they think that whatever they do wrong through ignorance, without any deliberate malice, but with good intention, is excusable.

But none of us would excuse the Jews for having crucified Christ, for having cruelly raged against the Apostles, and for having attempted to destroy and extinguish the gospel; and yet they had the same defense in which we confidently glory. Away then with these vain evasions regarding good intention; if we seek God sincerely, let us follow the only way by which we can come to Him. For it is better, as Augustine says, even to go limping in the right way than to run with all our might in the wrong direction. If we want to be truly religious, let us remember that what Lactantius teaches is true: that true religion is only that which is connected with the word of God.

And further, since we see that those who wander in darkness with good intention still perish, let us bear in mind that we are worthy of a thousand deaths if, after having been illuminated by God, we wander knowingly and willfully from the right way.