The Religion of Crypto-Jews

As a result, crypto-Judaism came to rely almost exclusively on three other sources for religious knowledge:

(1) Oral tradition. ...
(2) Old Testament, still readily available as part of the Christian Bible
(3) The Inquisition's Edicts of Grace. Because these public documents contained detailed lists of customs by which citizens might recognize their Judaizing neighbors.

Still, even with these sources of knowledge available, the Jewishness of crypto-Judaism rapidly attenuated.

- pg 40

As early as 1705 in an auto da fe a Portuguese bishop berated the Judaizers about to be reconciled for their paltry knowledge of Judaism and the scorn in which they were held by Jews elsewhere. The principal characteristic of these Judaizers was their self-identification as Jews. Unable or unwilling to engage in visible observances, they formulated a philosophy that claimed, as Kaplan put it, that “inner psychological identification with the Jewish religious and national heritage was more important than observance of the commandments of Jewish law”. Some affirmed the three key crypto-Jewish beliefs (one God; Messiah not yet come; salvation through belief in the Law of Moses). Some defined their Jewishness as negativism toward things Christian. Traces remained of the observance of the Sabbath, abstinence from pork, fasting, and Jewish funeral customs.

- pg 46

Secrecy and Deceit: The Religion of Crypto-Jews by David M. Gitlitz