Rambam Hilchos Mamrim 3:3
"To whom does the above apply? To a person who denied the Oral Law consciously, according to his perception of things. He follows after his frivolous thoughts and his capricious heart and denies the Oral Law first, as did Tzadok and Beitus and those who erred in following them.
The children of these errant people and their grandchildren whose parents led them away and they were born among these Karaities and raised according to their conception, they are considered as children captured and raised by them. Such a child may not be eager to follow the path of mitzvot, for it is as if he was compelled not to. Even if later, he hears that he is Jewish and saw Jews and their faith, he is still considered as one who was compelled against observance, for he was raised according to their mistaken path. This applies to those who we mentioned who follow the erroneous Karaite path of their ancestors. Therefore it is appropriate to motivate them to repent and draw them to the power of the Torah with words of peace."
Tinok shenishba - is the short term for : a Jewish child who was captured and raised by gentiles; therefore, they do not know anything about Torah, or even if they do, they are very estranged from it.
(See Talmud, Shavuos 5a.)
This term is usually and generally, extended to any Jew who is totally ignorant of Torah, or born into secular society estranged from Torah life. (Not just someone captured as a child. Although, modern Rabbis debate what exactly constitutes such status; ask your LOR.)
The above Rambam makes it clear that we should educate such people about Torah. The Rambam gives blanket direction to try and bring these souls back, despite any perceived risk of how they will eventually turn out if they take us up on the offer to return to the Jewish people in peace.
The Rambam does not specifically discuss "what if you think the person will fall back etc.", but he doesn't need to in order to be clear on the issue.
This is because it is obvious that many tinokos shenishbu will find it hard to return to Judaism by nature. Therefore if the Rambam declares a blanket rule to attract such people, without qualifications, it is tantamount to saying he is unconcerned with the potential of these people backsliding later. (In my humble opinion.)
I am not aware of any opposing opinion.
(This rule should not be confused with the famous rule brought in Talmud Beitzah 30, that we should not rebuke someone who we know will not listen, so that they should rather remain accidental sinners, rather than intentional sinners etc.
On this the Rema paskens in Shulchan Oruch Chaim 608:2, that such refrain from rebuke applies to Rabbinical decrees and Biblical decrees that are derived from deeper learning. However, if we see someone violating an explicit Torah law, written in plain language within the Torah text that is simple for all to know, we certainly must tell the sinner to stop even if we know they won't listen. However, it must be done tactfully. See the poskim there.
I believe there are more differences, however further discussion of comparing these two rules seems beyond the scope of this post.)
I hope this helps.