Answer: A human being is not excused due to ignorance if he is able to uplift this ignorance from himself and find someone he could ask. And also (he is not excused) with regard to a matter that is known from the Religion by necessity. And the likes of it must not be obscure – (such as when) a man deals with interest amongst the Muslims. So when it is said to him that this is Haraam (forbidden) he says: “I don’t know, I’m ignorant.” This is not acceptable because this is a matter that is clear to everyone.
However if he were to be in a community that deals with interest, and its example is obscure and hidden from him. And say no one there accepts Islaam for example, so he lives in America and the people there deal with interest, so he thinks that it is lawful, this person perhaps (he will be excused) due to ignorance. So when this person becomes a Muslim and continues to deal with interest, and says: “I don’t know, I’m ignorant” – yes, this person its possible that the likes of this person is excused due to ignorance. Meaning, if the ignorance is with regard to some matter that is precise and obscure – he is excused. As for with regard to the matters that are clear in which there are no obscurities – he is not excused. In the obscure matters that one can be ignorant about, i.e. such as the case with this individual, its possible that this person is excused due to ignorance.
This is like the story that has been reported in the two Saheeh Collections about the man that ordered his family (when he died) to burn his body and disperse his ashes in the sea and in the land, saying: “If Allaah is able to get hold of me, He will indeed punish Me with a severe Torment.” He did this out of ignorance. And what caused him to do this was his fear of Allaah, so Allaah forgave him and had mercy on him. 
He did not do it deliberately, thinking that he will not fall under Allaah’s Decree and Judgement in this situation. He neither rejected the resurrection nor did he deny Allaah’s Ability, however he did deny the completeness of the details of Allaah’s Ability because this was the extent of his knowledge. He did not deny this out of arrogance and stubbornness rather he was ignorant and the thing that caused him to do it was his fear of Allaah. So this is an example of an obscure matter, as it relates to him.
As for the clear matters, then what is not acceptable is when: A man lives amongst the Muslims yet he doesn’t pray. So when he is advised, he says: “I don’t know if the prayer is obligatory, I’m ignorant.” This person is not paid attention to. And likewise if he says: “I don’t know if alcohol is forbidden” or “if fornication is forbidden.” This person is not listened to in this regard since these are clear matters.
 Refer to this hadeeth in Saheeh Muslim (Eng. Book 37, no. 6638)
Posted from al-ibaanah e-Book:
Explanation of “The Nullifiers of Islaam” of Imam Muhammad bin Abdul-Wahaab – By Shaikh ‘Abdul-Azeez bin Abdilillah Ar-Raajihee
…May Allah give us a better understanding & guide us to accept the truth