Since it's the Holy Spirit's job to keep us in the faith...I guess the strength is His, not theirs.
That's one way to look at it, and I wouldn't disagree.
But I see more to it than just that.
It is said that God never gives us more than we can handle, but I consider this a serious error.
I would suggest that if it actually comes from God it will be more than we can handle and we have to ask him for the additional strength and power to add to our own to accomplish it.
So in that light, it is not the Holy Spirit alone that gives us the power, it is the combination of our own abilities exercised to their fullest in partnership with the Holy Spirit that triumphs over that which we cannot triumph by ourselves. God expects us to do our part.
But that's just my view as a result of my life, not to say that any other view is not equally as valid. We're all like the blind men feeling different parts of the elephant, none is right and none is wrong with all just having an experience of different parts of something much larger in its entirety.
The way I see it.