Assalamu alaykum. I read that Shaykh Ibn 'Uthaymeen said the following in Liqaa’aat Al-Baab Al-Maftooh (33/question no. 12):
"The excuse of ignorance is what is indicated by the Qur’an and Sunnah, as was stated by the leading scholars such as Shaykh Al-Islam Ibn Taymiyah (may Allah have mercy on him), and Shaykh Al-Islam Muhammad ibn ‘Abd Al-Wahhaab (may Allah have mercy on him). However, a person may be negligent in seeking knowledge, so he is sinning in that respect. In other words, it may be possible for him to learn, but he does not care to do so; or he may be told that something is haram, but he does not care. In this case, he is falling short in this regard, and he is sinning thereby."
What is meant with sinning? Does it mean that a person would be excused in the sense that he is not a disbeliever but would be sinning due to his negligence in searching the truth? May Allaah reward you.

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

Declaring a person a sinner because he neglects to seek the obligatory Islamic knowledge, investigate the truth, and ask about the correct religious ruling on an act after his attention was drawn to its unlawfulness and remains indifferent towards treading the path to salvation is clear in such cases. A person bears a sin for such acts. However, the intended scope of the label in this context is broader; it is to rule that he is not excused by ignorance, and he is subsequently ruled according to what he said or did as a disbeliever (Kaafir) or a dissolute (Faasiq).

Shaykh Ibn ‘Uthaymeen held that the matter should be further investigated. The person who is ignorant of a religious ruling should be judged according to whether his ignorance is excused or not. He addressed this topic in many of his books and lessons. He extensively and clearly detailed the topic in reference in his book Manthoomat Usool Al-Fiqh wa Qawaa‘idih while addressing the ruling on the one neglecting the duty of seeking the truth and falling short in seeking it. This book was written in the form of a poem composed by Shaykh Ibn ‘Uthaymeen along with his commentary on it. It was published under the supervision of the Shaykh Muhammad ibn Saalih Al-‘Uthaymeen Charity Organization. Shaykh Ibn ‘Uthaymeen wrote in it lines of poetry that mean, "The Sharee'ah provisions are binding only after acquiring knowledge of them, as evidenced by the fact that the acts done out of mistake or ignorance are excused. However, if the person falls short in learning the religious ruling in this regard, then his situation should be further investigated."

He commented on those lines, saying:

"This is a very important rule. Are the Sharee'ah provisions binding before acquiring knowledge of them? Scholars held different views in this regard; some maintained that they are binding as far as the fundamental rulings of Sharee'ah are concerned, and not the branches of jurisprudence. Other scholars held that they are binding in both, and some maintained that they are not binding in either. The most likely correct view in this regard is that the religious obligations and rulings are not binding before obtaining knowledge of them. Before obtaining knowledge of religious obligations and rulings, the person is not obliged to abide by them. This has been evidenced by many texts of general and specific indications..."

The Shaykh cited a number of these texts and then added:

"Based on this rule, we say that when someone does not perform a religious obligation without knowing that it is obligatory and has no one to ask in this regard, he is not required to make up for it... This is why scholars said that if a person embraced Islam in a distant place far away from Muslim lands and he left out a religious obligation, committed a prohibited act, or denied an indisputably established ruling of Islam, he is not to be held accountable for that because he is excused (by ignorance)."

About the line that says, "However, if the person falls short in learning the religious ruling in this regard, then his situation should be further investigated," the Shaykh commented:

"If the ignorant person could have learned the teachings and rulings of Islam in this regard, but he neglected it and fell short in acquiring such needed knowledge, then he is not excused because of his ignorance. This is because he could have removed this ignorance by learning, but he did not do so. If it is said that his situation should be further investigated, then that means that there cannot be a general rule in this regard. Each situation should be investigated, and the ruling on it should be decided in the light of the circumstances of that particular situation. For instance, if a person is proven to be neglectful in learning his religion, but the common knowledge in his community or country is that a certain act is permissible or not obligatory, then we do not judge that he is obliged to make up for what he missed because it did not occur to him at all that that act is obligatory. However, if he was in a place where he could have learned about his religion or in a place where it is common knowledge that an act is obligatory, but he neglects it and raises the slogan of lay-Muslims, 'Ask not about things which, if made plain to you, may cause you trouble', then we believe that he is obliged to make up for the obligatory act that he neglected. This is because the scope of the excuse of ignorance is diminished in this case, as underlined in the poem.

This view was highlighted by Shaykhul-Islam Ibn Taymiyyah in his book Al-Ikhtiyaaraat at the beginning of the Book of Prayer; it is a good opinion that has authoritative weight. For instance, when a boy sees a wet dream at the age of thirteen but does not perform the obligatory prayers nor fasts, believing that he only reaches puberty at the age of fifteen, do we then oblige him to make up for the prayers and the missed days of fasting? The answer is, 'The ruling depends on his situation; if he was utterly ignorant of the religious ruling in this regard, then we do not oblige him to make up for what he missed. But if he was neglectful in learning and educating himself on the matter, then the situation should be further investigated...'

Another question arises in this regard: if the ignorance concerns a matter that constitutes apostasy and Kufr (disbelief) if the person had knowledge of it; for example, if a lay-Muslim lived among people who call upon the dead and supplicate them and no one ever informed him of the prohibition in this regard and that such an act is an act of Shirk (associating partners with Allaah), and this person adheres to Islam and says, 'I am a Muslim,' would he be excused then for calling upon other than Allaah? The answer is: yes, he is excused because he has lived his whole life in that environment and no one ever explained to him that such an act is Shirk, and he believed that calling upon the dead is a means to draw closer to Allaah, not an end in itself. If this is the case, then this person is not declared a disbeliever, because he attributes himself to Islam, unless he is invited to follow the true religion of Allaah and rejects that under the pretext that there were more knowledgeable scholars who had not held such views on these acts. In this case, he has been educated beyond the excuse of ignorance. He stands in the same position of those who proclaimed blind adherence to the example of their ancestors in disbelief; Allaah, The Exalted, says (what means): {Rather, they say: Indeed, we found our fathers upon a religion, and we are in their footsteps (rightly) guided.} [Quran 43:22] Such people are not excused. He should have investigated the matter himself and sought after the truth on his own...

Some scholars maintained that the excuse of ignorance is not established as far as the fundamental rulings of Sharee'ah are concerned. They held that the ignorance of the one who engages in acts of polytheism is not excused, even if he still attributes himself to Islam. Were we to follow this scholarly view, then many Muslims nowadays would have been declared disbelievers! However, as for those who had Islamic knowledge or knew that such an act takes the doer out of the fold of Islam but insisted on performing it, following the example of those mentioned in the verse: {...we found our fathers upon a religion, and we are in their footsteps (rightly) guided.} [Quran 43:22], then those are, definitely, declared to be disbelievers. It cannot be argued otherwise." [Manthoomat Usool Al-Fiqh wa Qawaa‘idih, pp. 73-76]

In brief, Shaykh Ibn ‘Uthaymeen held that the excuse of ignorance is established in principle, except for the person who persists in committing a prohibited act or neglecting the obligation after being edified on it or the one who neglects the duty of seeking the obligatory knowledge and falls short in consulting with scholars despite his ability to do so and the existence of doubts about the lawfulness of such an act within him.

What emphasizes the view held by Shaykh Ibn ‘Uthaymeen in this regard are his statements and views underlined in many of his fataawa and books. For instance, he said:

"Any person who committed an unlawful act while being ignorant of its unlawfulness bears no sin, and no punishment is incurred as a result. This is because Allaah, The Exalted, is more merciful than to punish a person who did not intentionally disobey Him. However, the situation of that person should be investigated as to whether he neglected the duty of learning his religion and seeking the truth. If it is proven that he was neglectful in this regard and blindly followed the example of other people without seeking the truth and verifying the lawfulness of such an act, then he bears a sin for that; he bears a sin for neglecting the duty of seeking the truth, and his prohibited act may not be excused in this case. He may still be excused because of his ignorance if it did not occur to him that such an act could possibly be prohibited and he has no scholars who could warn him of such a thing; in this case, his action may be excused..." [Liqaa' Al-Baab Al-Maftooh]

Allaah knows best.