If the ignorant person is excused, can he be left in his ignorance?

Firstly: 

The excuse of ignorance is one of the matters subject to ijtihaad concerning which there is a difference of opinion among the scholars. Some of them say that the ignorant person is excused in all cases and some of them say that that is limited to certain matters. There is no room to discuss this issue in detail in this answer. 

Secondly: 

Among the scholars who say that the ignorant person is excused, there is no one who says that the ignorant person should be left without calling him or teaching him, because of the abundant evidence that it is obligatory to spread the religion, teach the ignorant and establish proof against those who are accountable. Allaah says (interpretation of the meaning):

“ ‘this Qur’aan has been revealed to me that I may therewith warn you and whomsoever it may reach” [al-An’aam 6:19] 

“Of every troop of them, a party only should go forth, that they (who are left behind) may get instructions in (Islâmic) religion, and that they may warn their people when they return to them, so that they may beware (of evil)” [al-Tawbah 9:122]

“Invite (mankind, O Muhammad صلىالله عليه وسلم) to the way of your Lord (i.e. Islam) with wisdom (i.e. with the Divine Revelation and the Qur’aan) and fair preaching, and argue with them in a way that is better. Truly, your Lord knows best who has gone astray from His path, and He is the Best Aware of those who are guided” [al-Nahl 16:125]

And the Prophet (blessings and peace of Allaah be upon him) said: “Convey from me even if it is one verse.” Narrated by al-Bukhaari, 3461. 

The Prophet (blessings and peace of Allaah be upon him) sent his Companions as teachers and conveyors of glad tidings and warnings. If leaving the people in ignorance was acceptable, the Messengers would not have been sent, the Books would not have been sent down and the callers would not have been charged with their duty. 

No one should imagine that leaving the ignorant in their ignorance is permissible. No rational person would say this, let alone any scholar. The fact that Allaah may excuse the ignorant person whose ignorance is not the result of negligence does not mean that the scholar should refrain from doing what he has been commanded to do of conveying and explaining. 

Allaah says (interpretation of the meaning):

“(And remember) when Allaah took a covenant from those who were given the Scripture (Jews and Christians) to make it (the news of the coming of Prophet Muhammad صلى الله عليه وسلم and the religious knowledge) known and clear to mankind, and not to hide it”

[Aal ‘Imraan 3:187]

It should be noted that if the ignorant person finds someone who can teach him, but he falls short and is negligent, then he is sinning because he has not learnt what he was required to learn and he is not excused, whether the matter has to do with beliefs, acts of worship or interactions with others. Hence Allaah criticised the heedless people who know about their worldly matters but are ignorant of their religion. He said (interpretation of the meaning):

“They know only the outside appearance of the life of the world (i.e. the matters of their livelihood, like irrigating or sowing or reaping), and they are heedless of the Hereafter” [al-Room 30:7]. 

And He criticised those imitators who ignore the knowledge and guidance that has come to them and say: we found our forefathers following a way and we are following in their footsteps. 

And Allaah knows best.