Actually, Lyle is the keeper of at least 2 secrets. However, the secret to which he is referring when he beseeches Victor Frankenstein not to share with others "the tribe into which he was born," he is most definitely referring to the fact that he is a Jew. This request is the bookend to what happens at the gate before they enter the "witches castle" together. At this point, Lyle, anticipating that he might die, recites the Shema, the Jewish declaration of faith and monotheism. When they end up surviving and emerging from the castle together, he is making reference to what he did and the Hebrew prayer he offered up earlier. Moreover, the use of the word tribe would definitely be an allusion to Judaism as we Jews are the descendants of the 12 Tribes of Israel. Finally, just to cement this point, because I think people have some misinformation about how antis elitism would've manifested itself at this time and in this context, Lyle would've feared discovery because, had anyone known he was Jewish, he would've likely experienced considerable backlash and his social circle and even his academic/scholarly life would've been seriously circumscribed. Many many institutions in Victorian England forbade membership and even admission to Jews. To your point about Disraeli, Benjamin Disraeli was indeed the Prime Minister of England, one of Queen Victoria's favorites, I believe. However, Disraeli's father, significantly, had converted to Christianity, and he was not raised in Judaism.