Can a first level cleric with an 18 wisdom activate a scroll with an 8th level cleric spell on it?
Provided the cleric succeeds on the DC 16 (minimum - could be higher) caster level check, yes. The cleric meets the requirements, but does not have the required caster level to activate the scroll automatically.
To have any chance of activating a scroll spell, the scroll user must meet the following requirements.
The spell must be of the correct type (arcane or divine). Arcane spellcasters (wizards, sorcerers, and bards) can only use scrolls containing arcane spells, and divine spellcasters (clerics, druids, paladins, and rangers) can only use scrolls containing divine spells. (The type of scroll a character creates is also determined by his class.)
The user must have the spell on her class list.
The user must have the requisite ability score.If the user meets all the requirements noted above, and her caster level is at least equal to the spell's caster level, she can automatically activate the spell without a check. If she meets all three requirements but her own caster level is lower than the scroll spell's caster level, then she has to make a caster level check (DC = scroll's caster level + 1) to cast the spell successfully. If she fails, she must make a DC 5 Wisdom check to avoid a mishap (see Scroll Mishaps). A natural roll of 1 always fails, whatever the modifiers. Activating a scroll is a standard action (or the spell's casting time, whichever is longer) and it provokes attacks of opportunity exactly as casting a spell does.
Our argument is based on this: at first level would the cleric have access to 8th level spell list.
I say she can because the 8th level spell is on her list. Our other player says she can't because she doesn't have access to 8th level spells yet.
(I hope I'm wording that in a way that makes sense.)
Yes. You have access to your entire spell list. You don't need access to spells of that level to use a scroll. That is the entire point of the caster level check. That would only need to be made if you haven't got spells of that level yet. Also:
Spell Completion: This is the activation method for scrolls. A scroll is a spell that is mostly finished. The preparation is done for the caster, so no preparation time is needed beforehand as with normal spellcasting. All that's left to do is perform the finishing parts of the spellcasting (the final gestures, words, and so on). To use a spell completion item safely, a character must be of high enough level in the right class to cast the spell already. If he can't already cast the spell, there's a chance he'll make a mistake. Activating a spell completion item is a standard action (or the spell's casting time, whichever is longer) and provokes attacks of opportunity exactly as casting a spell does.
The "chance he'll make a mistake" part is the required caster level check (and potential wisdom check if the caster level check fails).