Thank you for interpretative question, “What is the meaning of invincible ignorance in Rom 2:14-15?”
In order to discuss invincible ignorance, one must define the term. According to Catholic theology invincible ignorance excuses a person from all culpability. An action committed in ignorance of the law prohibiting it, or of the facts of the case, is not a voluntary act. The guilt associated with an offense committed in ignorance is less than it would have been if the act were committed in full knowledge, because in that case the offense is less voluntary.
Catholics and Protestants theologians differ in the interpretation of Romans 2:14–15. In order to evaluate this doctrine, one ought to consider the context of the verses.
The context says this, “All who sin apart from the law will also perish apart from the law, and all who sin under the law will be judged by the law. 13 For it is not those who hear the law who are righteous in God’s sight, but it is those who obey the law who will be declared righteous. 14 (Indeed, when Gentiles, who do not have the law, do by nature things required by the law, they are a law for themselves, even though they do not have the law. 15 They show that the requirements of the law are written on their hearts, their consciences also bearing witness, and their thoughts sometimes accusing them and at other times even defending them.) 16 This will take place on the day when God judges people’s secrets through Jesus Christ, as my gospel declares” (A searchable online Bible in over 150 versions and 50 languages).
As I read the context and text, invincible ignorance do apply to non-Christians. It is my understanding of the text that God does not judge a non-Christian based on the letter of the Law/Torah. The Apostle Paul argues that although non-Christians do not have the letter of the Law, they have in their conscience the spirit of the Law. Nearly every culture upholds that murder or thief is morally wrong. Thus even though the non-Christians do not know the Law/Torah, they instinctively have the spirit of the Law in their own conscience.
There are 613 command according to Jewish scholars. The non-Christians do not know all 613 commands of God but are conscious of the majority of the Ten Commandments except for the Sabbath Day. It is their conscience which will accuse or excuse the non-Christians on the day of judgment. Invincible ignorance does not excuse one’s moral conscience of knowing good or evil.
SUMMARY: Invincible ignorance is partially valid as to the letter of the Law but ignores the spirit of the Law of one’s culture and or conscience.
For more perspectives:
https://www.quora.com/What-is-the-meaning-of-invincible-ignorance-in-Rom-2-14-15