Atonement will not trigger with damage effects from trinkets and azerite traits.
Atonement healing is the only form of healing available to priests that requires a hostile target, and as such is the only form of healing not available between encounters. This also means the priest must be within range of the chosen hostile target in order to heal in this way. In some encounters, this can require the priest to move closer to the hostile target (especially bosses) than they would normally choose to, especially as these spells have only a 30-yard range (including [Penance], when used on hostile targets). Conversely, it is not necessary for the priest to be within range of the friendly target in order to heal them.
Atonement healing also requires the priest to be facing the hostile target in order to cast these spells but does not require the priest to be facing any friendly targets in order for them to receive healing. However, when using macros to attack a friendly target's target, the unpredictable nature of the hostile target's location can prove confusing, and can briefly interrupt the priest's healing.
Tips
When soloing mobs or players, Atonement is a great rotation for combined healing and DPS. Although the healing is halved when received by the priest, it is still enough to reduce or even remove entirely the need to shield or heal, saving global cooldowns and so potentially increasing DPS. However, priests using Atonement while under attack may still wish to use [Power Word: Shield], if only to prevent pushback.
Patch 8.3.0(2020-01-14): Atonement's healing has been decreased from 55% to 50% of damage done.
Patch 8.1.0(2018-12-11): Atonement's healing has been decreased from 60% to 55% of damage done.
Patch 8.0.1(2018-07-17): Atonement's healing has been increased from 40.1% to 60% of damage done.
Patch 7.2.5(2017-06-13): Atonement’s duration has been reduced to 15 seconds without the Contrition Talent.
Hotfix (2017-01-31):Atonement will now heal players who are out of line of sight of the Priest.
Hotfix (2016-12-13):Atonement effect increased by 10% in PvP.
Hotfix (2016-09-23):Atonement healing is now increased by 15% in PvP (down from 25%).
Patch 7.0.3(2016-07-19): Redesigned from a passive level 38 ability.
Previous: When you deal damage with Smite, Holy Fire, Penance, Power Word: Solace, or their multistrikes, you instantly heal a nearby injured friendly target for 100% of the damage dealt. If the Priest is healed through Atonement, the effect is reduced by 50%.
Patch 6.0.2(2014-10-14): Now heals for 25% less than before.
Patch 5.3.0(2013-05-21): Now heals nearby friendly targets for 90% of the damage dealt, down from 100%.
Hotfix (2013-03-05):Atonement healing granted from critical strikes now heal for the correct amount.
Patch 5.1.0(2012-11-27): Healing range increased to 40 yards, up from 15.
Patch 5.0.4(2012-08-28): Now also includes damage from Penance.
Patch 4.3.0(2011-11-29): Will now account for the target enemy's combat reach when calculating proper range, enabling it to be used on large creatures such as Ragnaros and Al'Akir.
Hotfix (2011-01-05):The heal from Atonement is now capped at 30% of the priest's health before any modifiers such as healing percentage bonus effects/talents.
Hotfix (2010-12-21):The healing effect from Atonement is now being affected by healing modifiers correctly.
[Trinity]
[Power Word: Shield], 
[Shadow Mend], 
[Power Word: Radiance]
[Penance], when used on hostile targets). Conversely, it is not necessary for the priest to be within range of the friendly target in order to heal them.
