As in most of Latin America, Roman Catholicism is the predominant religion in Argentina, and there is at least one church in every Buenos Aires neighborhood to accommodate the faithful. Very few of them, however, offer services in English. One church that does is Madre Admirable in Retiro (Arroyo 917, phone 4393-3887), with an English-language mass each Sunday at 10am. Specific mass schedules throughout the city can also be obtained through this church locator: http://www.horariosdemisa.com.ar/english/index.html
The Anglican Cathedral of St. John the Baptistis an Anglican church that holds mass in English every Sunday at 9:30, except for the fourth Sunday every month when they hold the mass at 10:00. It is located at 25 de Mayo 282, just three blocks from Plaza de Mayo.
International Bible Church is located at Sarmiento 618 in Martinez, just outside the city limits. It has English services on Sunday at 11am, and prayer meetings on Wednesdays, but it is advisable to call ahead for the Wednesday meetings: 4792-4330.
United Community Church is located at Av. Santa Fe 836 (also in Retiro), and has English services on Sundays at 10:00, as well as bible classes and other events in English.
Templo Libertad, the oldest synagogue in Buenos Aires, is located at Libertad 769 in Tribunales. They are very welcoming to foreigners, with services held in Hebrew.
Centro Cultural Islámico Rey Fahd is claimed to be the largest mosque and Islamic cultural center in Latin America. Website in Spanish; they do not currently hold worship in English. Located in Palermo near the polo grounds (Av. Bullrich 55, phone 4899-1144), it is the only mosque in Buenos Aires at this time.
There are several Buddhist centers and organizations in the city, such as Kagyu Tekchen Choling at Av. Melian 2171 in Belgrano (phone 4543 4925), with meditation services and classes in English.
It is also a good idea to check the Sunday edition of the The Buenos Aires Herald, as sometimes congregations will post their English services under the classifieds section.