When I studied composition in the 1960s, it was still forbidden to write in parallel fifths. So listening to the type of free voice leading musical esoterica presented by the Tallis Scholars Wednesday evening as part of the Mostly Mozart Festival can only be classified as a guilty pleasure. We owe our cultural heritage to the codifiers of music during the Enlightenment, but much that was beautiful and deeply spiritual was lost in the zeal of regimentation.

Shuffling themselves at each interval like players in Stravinsky’s ballet Jeu de Cartes, these 10 singers dazzled with their cleanliness of sound, each acting in turn as a human pitch pipe to ground their colleagues for the next auditory endeavor. The music that they explored is so old that it sounds ultramodern, putting me in mind of the choral music of Anton Webern, who wrote his doctoral dissertation on one of this evening’s featured composers, Heinrich Isaac. It is thrilling to hear these deliciously dissonant passages and easy to see the genesis of such daring leaps as those that color the most adventurous 19th-century music, the Adagio from Bruckner’s Symphony # 7, for example.

The group opened with Isaac’s pleasant motet, “Innsbruck, ich muss dich lassen,” written as he took his leave of his first significant post. These extraordinary musicians then launched into a complete setting of the Mass.

Regular readers of this column will remember that, in connection with the Festival of Sacred Music at St. Bartholomew’s the other week, we discussed the music of Hans Leo Hassler and its relationship to the unique architecture and acoustics of St. Mark’s cathedral in Venice. This evening, we heard Hassler’s Missa VIII for Double Choir, an antiphonal masterpiece straight out of the Gabrieli school.

The scholars rearranged themselves, switching from the placement used for the Isaac (boys on one side, girls on the other) to two quintets of voices that were mirror images of one another. This allowed for the magical, “follow the bouncing ball” effect of melodic peregrination and for the inclusion of several interweaving rounds. Apparently the concept of a Mass is foreign to many, as the audience kept applauding in all of the wrong places. Clapping after a Mass is problematic enough, but another sea of ovation drowned the last quiet, peaceful syllable of the dona nobis pacem. For the spiritually bereft, here’s a tip: When the conductor turns around to face the audience and the choristers close their books, only then it is a good bet that the piece is over.

The last work before intermission was Virgo prudentissima, an Isaac Marian motet. Here I thought of Webern’s late Cantata No. 2 and marveled at how 450 years separated the two pieces. As in every effort of this evening, the Tallis Scholars proved they have established the gold standard of a cappella tone painting to which all religious choirs should aspire. This dedicated group’s quixotic retreat to a church in the English countryside for concentrated, totally immersed rehearsal has produced some of the finest singing in memory.

Do you know the story of the young Mozart attending a service at the Sistine Chapel and then writing down from memory the complex music that he had heard inside? Well, that same piece, the Miserere of Gregorio Allegri, opened the second half of the program. Perhaps I should rather say a variant of the piece, as it has been revised several times since Mozart’s day. For this intensely celestial exploration, only five singers came out on stage. As the work progressed, one high male voice intoned from the rafters stage right, followed by the quartet in the upper balcony stage left. The overall effect, especially the ethereal high C from soprano Deborah Roberts, who composed her own ornamentation, was thrilling.

Finally, the Tallis Scholars presented the work of the German Magnificat of Heinrich Schuetz, whose Seven Last Words of Christ may be the most profound music composed before Bach. The jarring effect of consonance and tonic to dominant relationships was illuminating, and brought us much closer to the music that would pave the way for Mozart himself. For me, this concert was the highlight of the summer.

Until August 26 (Lincoln Center, 212-721-6500).