Well, you may certainly not receive Communion in a Protestant service or participate actively in what may be considered the equivalent of our sacraments; this is what the Church calls communicatio in sacris—sharing in sacred rites—and to participate actively in them would imply adhesion to Protestantism and departure from the Catholic faith. This prohibition is considered by most traditional theologians to be of divine origin. In fact, the former discipline of the Church went further and prohibited any attendance at Protestant services, except for those services attendance at which is required by civil comity, such as weddings, funerals, baptisms, and the like, on the grounds that, though not strictly forbidden by divine law, such attendance could be dangerous to the integrity of the faith of the Catholic party, a prudential judgment on the part of the Church. Now, however, this latter prohibition is no longer in force, and Catholics may attend non-Catholic services, provided that they do not participate in any aspects of the service which are contrary to the Catholic faith—and, of course, is the service is filled with anti-Catholic elements, attendance would be problematic—and that in their considered judgment the spiritual good coming from such attendance will outweigh any possible harm. This is called communicatio in spiritualibus, sharing in spiritual things. Also, it would be unwise to attend non-Catholoic services so often as to give rise either to the reality or to the appearance of indifference.