This interesting talent increases your healing by 4/8/12% on targets below 50% health. 12% is a large bonus for 3 points, but is the 50% health requirement a good thing or a bad thing?
I think there's two ways to look at this talent. One the one hand, you could say Test of Faith is a bad talent if people are doing what they're supposed to, because if they don't stand in the fire or botch their abilities, they will rarely be below half health. On the other hand, you might say Test of Faith kicks in when you need it most, when people are in dire need of healing. And should you really pick your talents to help when everything is going fine, or to help you most when the raid needs all the healing it can get?
I think that both arguments are valid, but that there are two extenuating factors. First is that many of us are learning new bosses and new hard modes in Ulduar. This means that everyone is still learning where to stand, what to click, and how to avoid damage. People will be taking more damage as guilds learn encounters, so Test of Faith can be a good safety net. Also, there is a lot of raid damage in Ulduar, much of it nontrivial. Ignis's Flame Jets and Kologarn's Shockwave are two examples of raid-wide abilities that will bring many people below 50% health. Test of Faith can add a lot of oomph to the CoH's, PoH's, and PoM's that will be flying around.
Of course, the decision to invest in any talent comes down to what else the points could be spent on. This may be a number of other Holy talents but for me, I was comparing it to Blessed Resilience, which gives a flat 1% boost to all healing per point. Test of Faith gives 4 times the bonus per point, so you could look at total throughput and ask if Test of Faith will proc on your heals at least 25% of the time. If so, then it will affect your bottom line more than Blessed Resilience. Of course, the times Test of Faith procs are times when people need healing most, and when heals are least likely to overheal. So even if it only procs on 15-20% of your heals, perhaps it is still better overall.
Although I was skeptical of this talent early on, I have decided to use it. The +12% healing is a strong bonus, and given all the damage flying around in Ulduar, and the fact that our smart heals (CoH, PoM, DH) seek out injured players, it seems a good investment to me.