Therefore (toigaroun). This old triple compound particle (toi, gar, oun) is in the N.T. only here and Heb 12:1. Paul applies the logic of the case.
He that rejecteth (ho athetōn). This late verb (Polybius and lxx) is from ȧthetos (a privative and verbal of tithēmi, to proscribe a thing, to annul it.
But God (alla ton theon). Paul sees this clearly and modern atheists see it also. In order to justify their licentiousness they do not hesitate to set aside God.