Mining difficulty adjusts automatically. There is a price at which Bitcoin is no longer secure from the entity willing to spend the most money to 51% attack it

Mining difficulty adjusts automatically. There is a price at which Bitcoin is no longer secure from the entity willing to spend the most money to 51% attack it. But there's no good way to know how big that threat is. Most likely Bitcoin has a significant safety margin there (I'd take a rough guess at >100x).