Thousand Sons Sorcerers start with 15 Corruption Points and aligned to Tzeentch. It is feasible that after the first Compact, the threshold for checking aligment, i.e., 20 Corruption Points, is reached. However, what would the Alignment be if no Advances were purchased for any particular god (for example, all the XP was either spent on obtaining several unaligned psychic powers, biomancy, pyromancy, telekinesis, telepathy, divination, etc.)?
Page 75 of the Core Rulebook on Checking Alignment states: "[…] compare how many Advancements they have purchased that are affiliated with each Chaos Power". So, purchasing from unaligned is irrelevant to checking Alignment. I can't seem to find anything in the Tome of Fate book that would clarify how to treat such a case.
Therefore, at the 20 CP threshold, the Alignment count in the above example for each of the 4 gods is 0. So, does the Sorcerer become Unaligned or remain aligned to Tzeentch? What if less than 5 Advances towards Tzeentch were purchased? Tzeentch or Unaligned? What if less than 5 Advances towards Khorne, Nurgle, and/or Slaanesh were purchased? Tzeentch or Unaligned?