Of Johann Sebastian Bach’s huge legacy of compositions, his two surviving Passions – the St. Matthew and the St. John – are among the most ambitious in scope, grandest in scale and best-beloved among fans of choral music. The better-known St. Matthew Passion employs a double orchestra, a six-part choir and runs about 45 minutes longer. The St. John Passion, which will be performed at Bard College’s Richard B. Fisher Center for the Performing Arts this weekend, is almost a chamber piece by comparison. It calls for “only” a four-part choir and a smaller orchestra that in Bach’s time included what are now regarded as “ancient instruments”: oboe da caccia, lute, viola da gamba and viola d’amore, all known more for their intimate tones than for their dynamic power. Thus the singers have to carry more of the heft of this sacred oratorio.