I was just told by a Protestant that prior to Vatican II it was a sin for Catholics to go to a Protestant church service. Is this true?
Let’s begin by sifting out whether it was a sin from whether it was a permissible action. Again, sinfulness depends on the individual’s knowledge and consent; the action itself can only be determined permissible or not (or moral or immoral).
The common belief of average Catholics of the pre-Vatican-II era was that a Catholic could never attend a Protestant service. This understanding was flawed, but it was based upon the Church’s much more restrictive position on Catholics attending non-Catholic services.
The 1917 Code of Canon Law stated that, ordinarily speaking, a Catholic should not attend non-Catholic services; however, the Code did provide for exceptions to the normative law:
§1 It is not licit for the faithful by any manner to assist actively or have a part in the sacred [rites] of non-Catholics.
§2 Passive or merely material presence can be tolerated for the sake of honor or civil office, for grave reason approved by the bishop in case of doubt, at the funerals, weddings, and similar solemnities of non-Catholics, provided danger of perversion and scandal is absent (canon 1258, 1917 Code, emphasis added).
Basically, if a Catholic had a serious reason to attend a non-Catholic service (e.g., he was a convert whose non-Catholic relative had died), he could attend the service as a guest, not taking part in the service but merely observing it. For him to attend such a service without serious reason was considered grave matter because it was believed to jeopardize his Catholic faith:
Whoever in any manner willingly and knowingly helps in the promulgation of heresy, or who communicates in things divine with heretics against the prescription of canon 1258, is suspected of heresy (canon 2316, 1917 Code, emphasis added).
The law was strict in this era because it was a time when one’s religious observance was considered to denote his religious identity. For a Catholic to attend a non-Catholic service without serious reason was for Catholics of the time considered to be a betrayal of Catholic identity. Still, as can be seen from the exceptions above, the law was not absolute. Consider for example the case of St. Edith Stein, an early-twentieth-century saint who was known to have attended synagogue with her elderly mother after her conversion to Catholicism from Judaism. St. Edith attended Jewish religious services out of respect for her mother, but prayed the psalms out of her Catholic breviary (source).
When considering why the Church was strict on this in the past, we must remember that Protestants of the time were more likely to actively seek to evangelize Catholics because often they believed Catholics to be non-Christians in need of salvation according to the Protestant understanding of the gospel. Avoiding non-Catholic services was both a matter of not giving scandal to fellow Catholics and protecting one’s own faith from aggressive and often anti-Catholic proselytization.
Even today, while the common understanding of Catholics is that they are permitted to attend non-Catholic services, the Church places some strictures on Catholic attendance at non-Catholic services. Compare the prior canonical prescriptions with the canonical prescription of the 1983 Code:
One who is guilty of prohibited participation in [non-Catholic] religious rites is to be punished with a just penalty (canon 1365, 1983 Code).
For information on permitted and prohibited participation in non-Catholic Christian services, please see Directory for the Application of Principles and Norms on Ecumenism.