This card is excellent in a Preston Fairmont deck, taking advantage of his base value of 1 in each skill. Since he's likely to be mitigating those with cards like Dark Horse and Lola Santiago, Preston can use this on a skill test that he's likely to pass and still put several tokens in the bag.
Commit only to a skill test you are performing, and only if the difficulty of that test is higher than your base skill value.
After you commit Signum Crucis to a skill test, add X tokens to the chaos bag. X is the difference between the test's difficulty and your base skill value.
Repeatable with Silas on high shroud locations for a steady stream of lots of bless tokens. Since it takes effect after being committed rather than after resolution, Silas can reap the benefit turn after turn. Maybe you'll even start succeeding on some of those investigates!
Double Quick Learner + double Drawing Thin + Signum Crucis on a base shroud 4 for example adds 10-[base intellect] bless tokens(you might miss the fact that it's still comparing to your base value, your passive buffs or commited icons do not change this). Pretty sick.
And why would I wanna do this you ask? Because Ancient Covenant is extremely strong, and the more people in your team that can take it the more attractive this gets.