The magic of this work rests in the individual parts sung by each of the 40 voices. A lesser composer would have concocted a bombastic mess, but Tallis deploys the voices artfully. The main vocal line passes from choir to choir, slowly piling strata upon strata of grandiose sound, which then subsides after a phrase or two. At the end of the piece, all 40 voices converge majestically, as if God had preordered a version of Ravel's Bolero but with angelic voices instead of the tick-tick-tick tidal wave of accumulating orchestral instruments.
One tip: Forget about following the Latin text. Choral composers of yore generally enjoyed the advantage of setting a well-known passage. They could expect that their educated listeners understood Latin and probably knew the text already. The uneducated were merely obliged to be tremulously worshipful. Today, we can revel in Tallis' astonishing layers of sound without the distracting gravity of meek religious belief.
A final note: if you're looking for the sublime in sacred vocal music, I could not recommend more heartily the boxed set Vivaldi: The Complete Sacred Music. Snap this one up folks!