Complex sentence: Resistance was so strong that in 1615, John Sigismund backed down and relinquished any attempt at forcible conversion, allowing his subjects to be either Lutheran or Calvinist according to the dictates of their own consciences.
Split into simpler sentences: Resistance was so strong that in 1615, John Sigismund backed down and relinquished all attempts at forcible conversion. Instead, he allowed his subjects to be either Lutheran or Calvinist according to the dictates of their own consciences.