If a spell states to add "spellcasting ability modifier" do I add just the modifier (wis or int) or the full spellcasting ability (wis + proficiency)

Question

Spiritual Weapon states damage is "1d8 + your spellcasting ability modifier". As a Cleric my "spellcasting ability modifier" is wisdom with +3.

Does this really mean I only get 1d8+3 since it specifies "modifier" instead of just "spellcasting ability" +5?

Answer 1

You're conflating two separate things.

Spell attack bonus is to hit the target, which gives you a bonus of your spellcasting ability modifier (Wisdom in your case) plus proficiency. That's the "melee spell attack" referred to in the spell's description.

The damage done by the spell if it hits is 1d8 plus your ability modifier (Wisdom again). So yes it's "only" 1d8+3 for you right now. But remember, spiritual weapon uses your bonus action to make an attack/move, meaning you can attack with it and your own attack, or attack with it and cast a spell, or use an item, and so on, and all without requiring concentration to maintain the spell. It's pretty powerful in that regard.

Answer 2

You only apply the modifier

Spiritual Weapon states the damage as (per PHB p. 278):

1d8 + spellcasting ability modifier

The spellcasting ability modifier is simply the ability score modifier for that attack - in this case, Wisdom, at your stated +3.

You can increase the damage dice by casting this at a higher spell slot.

When you cast this spell using a spell slot of 3rd level or higher, the damage increases by 1d8 for every two slot levels above the 2nd.

This is different than how you determine a successful attack

The use of proficiency bonuses never applies to damage - only to attack rolls.

Determining a successful attack involves rolling a d20 and adding both your ability modifier and proficiency bonus. Rules around this can be found on page 193 of the PHB.