Earlier quoted context omitted.
I am wondering what the most serious consequence will be, resign or more serious?
There will be no consequences because Congress is behind this program, it is legal, and whether or not this testimony was a "lie" depends upon very specific definitions of terms like "intercept". Most of us might hate that the government has this power but that does not make it extra-legal.
Still, a resignation might give us some hope that at the very least the recognize how outraged this makes use, and they'll try to do better in the future. If they don't do that, then that's just defiance against the American people and against the Constitution.