I found that my spell casting proficiency increases for my cleric even when I multiclass as a fighter.
The Divine Strike description says "When you reach 14th level, the extra damage increases to 2d8." and does not explicitly say I have to be 14th level of cleric. If I am 11 levels Tempest Cleric/ 3 levels Eldrich Knight = 14 levels total, will I get the 2d8 damage with Divine Strike?
You have to have 14 cleric levels. Class features require levels in the class, and don't benefit from levels in other classes except when otherwise stated. The reason that your spellcasting improves when you multiclass with fighter is that the rules for multiclassing are one such exception, explicitly counting one-third of your fighter levels (assuming you're an Eldritch Knight figther, of course, and rounded down) toward the Multiclass Spellcasting feature (p. 164).
To be doubly sure, the PHB restates at the beginning of the Multiclassing section on Class Features (ibid) that you gain class features from class levels, with only a few explicit exceptions to that rule:
When you gain a new level in a class, you get its features for that level. A few features, however, have additional rules when you’re multiclassing…
Another notable exception to this rule is the improved versions of cantrips, which explicitly work off your total levels rather than your spellcasting levels.
