It works like this:
When you activate it you make a glowing circle on the ground. You get a flat +15% damage bonus versus everything no matter if you are in the circle or not. If you also stand in the circle the little "Maker's Wrath" icon shows up as well, which means you get +10% bonus versus demons on top of this.
So: +15% versus everything, +25% versus demons if you stand in the circle.
If this is true, the skill is excellent, as it's essentially a 60% cumulative damage buff, with 45% of it applied to party members whose DPS should be far greater than that of the casting templar. For players trying to choose between Cassandra and Blackwall for their tank, this could be the clinching skill, especially later in the game, when both can become invincible.
My comment about conflicting feedback still stands, though, since even the posts in this thread are all over the place. This skill should be easy to test for anyone who unlocked the specialization:
1. Solo a non-demon enemy with a Templar to avoid interference from party members and record a few damage numbers from basic attacks. The more, the merrier, to avoid RNG.
2. Drop the buff and move a fair distance away from the circle. Continue attacking the same enemy and record damage numbers.
3. Average the numbers from step 1 and step 2 and compare the averages. If the number from step 2 is about 15% greater than that of step 1, then the skill works as Jais9 mentioned.