Idols are objects received by using Nate Dragon (some objectives are unavailable, use Fire Phoenix or Merci, the Mad Wizard instead), Kizlblyp, the Alien Traitor or Robo-Rudolph (the last two are event crusader, only available if unlocked) reset.
Each one gives an additive boost of 3% to DPS, and 1% to gold drops. Some Campaign Objectives are locked until a required number of idols has been met.
Idols can be spent on talents, and spent idols will continue to provide their boost to DPS and gold.
Initially, idols can be received for every 500 crusader levels on reset, but with Efficient Crusading and Deep Idol Scavenger talents, you can receive even more.
With Efficient Crusading (Tier 2) talent upgrade, crusader levels required for idols reduced, for 10 per level. Maximum level is 25, i.e. reducing the required level by 250 to 250 levels.
With Deep Idol Scavenger (Tier 3) talent upgrade, you can earn extra idols by unlocking more area, for 0.02% per area per level. Maximum level is 25, i.e. increase the idol gain for 0.50% per area.
You can also watch a small ad to get 25% more idols during reset, but this is not available for Steam because Steam don't allow ads.