Tiefling Mystic Theurge? how does it work?

[Jackofalltrades]

So i have read some treads and it appears that a Tiefling can enter Mystic Theurge 2-3 lvs early, so i would go Tiefling cleric 5/Mystic Theurge 15, but i don't understand something, the Mystic Theurge gains +1 lv of existing divine/arcane spell casting class. sooooooo....it would continue my divine casting, but what about the arcane casting? i don't have any preexisting arcane caster lv. do i miss out on that? do i suddenly gain arcane spell casting. the only way i'm meeting the 3rd lv arcane spell is through the tiefling's built-in darkness spell. any thoughts?

[grarrrg]

The early entry method has been ruled out of existence (technically FAQ'd out, but whatever).

When it did 'work', you still needed to take one actual level of the other casting class for the spellcasting to apply.
(also, there are only 10 levels of Mystic Theurge)