I am a cleric who recently had their holy symbol stolen from them (my backup was in my backpack). I needed to cast a spell. From my list of prepared spells I had summon monster, which has both a focus and DF. As a divine spellcaster am I allowed to cast the spell using its focus instead of the divine focus?
To cast a divine spell that possesses a divine focus (DF) component, a typical caster must employ a divine focus even if the same spell also possesses an arcane focus component
On Magic on Components on Divine Focus says
If the Components line [of a spell] includes F/DF or M/DF, the arcane version of the spell has a focus component or a material component (the abbreviation before the slash) and the divine version has a divine focus component (the abbreviation after the slash).
In other words, a typical divine caster cannot opt to use the arcane focus instead of a divine focus. For example, a typical level 1 cleric must have a holy symbol to cast a summon monster spell, while a typical level 1 wizard must have a tiny bag and a small candle.
Note: A spell's Components entry can be confusing because sometimes a spell (like the summon monster spells) seems to require a divine focus other than a the cleric's typical divine focus, when, in actuality, the spell doesn't. For example, a summon monster spell's components of V, S, F/DF (a tiny bag and a small candle) include only the necessary arcane focus, boldly assuming the reader knows that the divine caster will need, instead, his divine focus. However, some spells mandate only a focus (e.g. the destruction spell), and both arcane and divine casters must employ such a focus! This isn't helped by a spell like scrying that's just all sorts of confusing. In fact, I find a spell's whole Components entry an alienating jargon issue that I hope is one day cleaned up for ease of use.