
A divine being has bestowed upon you a unnatural affinity for healing, it is from this that you possess a "Radiance of Life" this is a aura of sorts that radiates outward from you and heals any allies or good-aligned creatures within 15-feet or within line of sight whichever applies. The healing is equal to the creature's constitution modifier for each round with a "minimum of 1"
Should you act of your own will in a manner that is evil the "Radiance of Life" will not work until you have done something to correct or reconcile your evil act.
The healed amount will deal radiant damage to undead with no a save, this Radiant aura does not require an attack roll. However the creature must be within 15 feet of the wielder of the aura to take the damage from the aura, resistance towards radiant magic will also reduce the damage the undead would take. "You cannot deal damage from the aura to undead by line of sight the undead must be within 15 ft"
- Whenever you roll dice for an ability or spell meant for healing hit points, re roll 1s rolled for healing hit points. You may only re roll once.
- Whenever you heal a creature at least 1 hit point with a level 1 spell or higher. That creature gains resistance to a damage type that you choose until the end of your next turn. If a spell heals more than one creature, this damage resistance will apply to only one of the creatures of your choice.
- Whenever casting a spell as a bonus action that heals hit points, you may cast an additional spell. However the spell must also be a spell that will heal hit points or remove a condition and casting the additional spell will not grant damage resistance.
- Whenever a healing spell is cast, roll a d20.
- If you roll a 1, you do not expend the spell slot used to cast the healing spell.
- If you roll a 20, The spell heals the maximum amount of hit points at the level cast plus 1d6 + 2x your wisdom modifier of temporary hit points and the spell slot is expended.
- If you roll a 2-19 the spell slot is expended.

This is way too strong.
Firstly, allowing the casting of another spell that is not a Cantrip is massively broken. With this feat, a healer could cast a 3rd level mass healing word, followed by a 5th level mass cure wounds. Assuming a 4-player party, they would restore an average of 80hp in a single round (assuming a 16 wisdom score).
That's without taking into account the free 15ft heal every round, the chance to not consume a spell slot and to heal at max hp, the resistances, and the re-rolling of the healing dice.
This thing is like 4 feats rolled into one.
well it is a homebrew feat. if you dm and want to take a ability out of any feat or give the player the option to pick part of it instead you can always do that. or you can always allow a player to pick part of the feat and then take it again or twice more to completely level it up and get all the abilities with it for example.
this feat was made with intent of long or high level/high difficulty encounters in mind for experienced players in more difficult campaigns. nothing is over powered if a dm balances feats or players with their campaign. it always comes down to how a dm will incorporate feats. this feat was used in a high level campaign focused around the curse of strad and another very difficult home brew campaign and it worked fine. it depends always on how the dm balances the abilities of the players with the difficulty of the story and how quickly you allow your players to level up. if your playing a slow leveling campaign and they are specializing in something i say reward your player in what theyve been focusing on they are still limited to healing if that mostly all they have been focusing on.
it was also made for healing type characters not fighters. in my mind if someone only focuses on one type of thing they become specialized in that one thing. and this feat is meant to be for that type of character.
i noticed in dungeons and dragons the standard feats and abilities for healing are lacking compared to other role playing games when you get to high levels. such as hero system for example in that game and this makes sense you do not unlock feats your character can get better at something and learn skills by practicing at that one thing just as you would in real life practicing at anything you do. by this system you can specialize in something because you have the ability to put skill points in something you have been practicing to continually make that skill even better. and in dnd if someone wanted to specialize in only healing and eventually unlock this there is still plenty of things the character cant do without the rest of the party. this means that a character can progress in something they focus on but not something they arent focusing on improving. if you focus on specializing in one thing you can eventually become a master at it. rather that be healing, smithing, sneaking, etc.
in my experience in a high level campaign this isnt any more overpowered than some other abilities or feats that are out there as usually players that focus on healing are limited to what they can do in combat anyway as far as dealing damage. which is why the minimum wisdom score is required because unless you are really lucky when rolling up your characters you wont have really high stats and if you put your highest stats in dex or strength for example it might be difficult to unlock this ability in the first place if at all.
also if all of your characters are focusing on different things they can balance each other out and each serve a purpose in a party.
over powered maybe if you were able to get this at level one but that would depend on the dm and or the difficulty of the campaign in mind. over powered is a term relative to how a dm balances the campaign and how difficult they have made the campaign in conjunction with how the party is balanced out. I hear what your saying though for a low level campaign and to get that early on i agree. but it can always be up to a dm on when to allow the character to gain this feat and for a difficult campaign it is fine plus again it depends on the dm how they want to allow this feat or how they balance the campaign. it can work depending on the type of campaign that is being played.