First of all, the alleged "calculation" is useless, since it is based on no knowledge of actual processes required for "complex chemistry" to become recognized as "simple life".
But second, the number of single celled critters on earth today is circa 5 times ten with 30 zeros, each of which can multiply tens of thousands of times per year (104th), over billions of years (109th), and now we're looking at 1043 reproductions = opportunities for descent with modifications and natural selection to operate.
Of course anybody can claim that such-and-such is impossible or improbable, but they will need more than flawed mathematics when physical evidence shows it obviously did happen.
As for God's role in all this, well Who do you suppose built the stage on which life's many dramas play out, in the first place?
And do you suppose the Director of life could not, from time to time, move His characters around on the stage, or replace them, so as to more clearly tell His-story?
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You cannot create a mathematical model of a process that is not completely understood. So far nobody can lay claim to understanding how it all works, much less having constructed an accurate mathematical modeling of it.