A big X-class solar flare would do the job. If one as powerful as the Carrington Event of 1859 happened today it would result in the deaths of billions of people.
So, what is the likelihood of this happening?
"The 2021 Astrophysical Journal study analyzing Kepler data suggested that superflares about 10 times more energetic than the Carrington Event may happen about every 3,000 years, and ones about 100 times more energetic may occur about every 6,000 years."
Yeah God is in control of everything- allowing, disallowing etc- He could use natural or man-made disasters or attacks-