Short answer: It says equal to his Craft skill. His Craft skill is his Intelligence (a d10) +3. Any check boxes you've put in his Intelligence will of course add to his Craft.
We're obviously talking about the Skull and Shackles version of Damiel, who has the power "When you play a spell that does not have the Attack trait, you gain the skills Arcane and Divine equal to your Craft skill until the end of the step"
First of all, the FAQ for Skull and Shackles amends the wording of this power to read "You gain the skills Arcane and Divine equal to your Craft skill while you play or when you would banish a spell that does not have the Attack trait". Much better wording!
There are few non-Attack Skull & Shackles spells where having good Arcane or Divine skill matters. Most S&S non-Attack spells just give a set bonus, reduce damage, or some other utility effect. The two examples I can think of are Animated Weapon and Telekinesis. In these cases, Damiel would replace Arcane or Divine with his Craft skill.
Why this power of Damiel's matters most, however, is when he would banish a non-Attack spell. Usually, if you play a spell and don't have the appropriate skill (Arcane and/or Divine), the spell is banished back to the box. ("After playing this card, if you do not have either the Arcane or Divine skill, banish it.") It's like the character used a scroll, and now it's gone. Damiel's power, however means that he does have the skill and can try to recharge the spell, and even if he fails, the spell is simply discarded, not put back in the box.
I hope this was clear.
Things work a little differently with Core rules. If you need any help with those, let us know.
I have doubts regarding these 2 spells, especially regarding Animated Weapon. Damiel's skill is active only "WHILE you play a spell", and "while" is not the same as "while and after". So, as soon as you resolve the spell by playing a weapon, you finish playing the spell and your Arcane and Divine skills are back to normal d4.However, I cannot find in the rulebook what happens if you've already added your skill to the check (meaning the die and modifiers) and then your skill suddenly changes. It is unclear if you must replace the added die and modifiers to their new values.
The spell says "for your combat check" which is a duration (given that checks/steps are demarcated game periods), so the spell is in play until that combat check is resolved, so therefore Damien's "while you play a spell" is fulfilled until the end of the check.
I imagine the text glowing like the inscription on the One Ring while any of the text of its power is still relevant to play, which I find a useful visualization when ruling the "out of your sight" rule.
It might be more debatable under the original wording of "when" but Dark Nero's "only update the check skill at the check check skill step" interpretation would be applicable there too.