Learn Finnish: Minun pitää vastustaa tätä houkutusta.

This is a necessive sentence expressing obligation. When the verb pitää is used to mean "must" or "have to", it requires the subject to be in the genitive case (minun) and it always stays locked in the 3rd person singular, regardless of who is doing the action.

Objects in necessive sentences are usually in the nominative case. Why is tätä houkutusta in the partitive?

You are right that total objects in necessive sentences normally look like the nominative (e.g., Minun pitää avata ovi - I must open the door). However, the verb vastustaa (to resist) is a partitive verb. When a verb strictly requires a partitive object, that requirement overrides the special object rules of necessive sentences.

Where does the noun houkutus come from?

It is derived from the verb houkuttaa (to tempt, to entice). Adding the -us or -ys suffix to a verb stem is a very common way to create an abstract noun in Finnish, transforming "to tempt" into "a temptation".

Could we use the verb täytyy instead of pitää here?