A.T. Robertson Commentary 1 Thessalonians 4:8

"Therefore he that rejecteth, rejecteth not man, but God, who giveth his Holy Spirit unto you." — 1 Thessalonians 4:8 (ASV)

Therefore (τοιγαρουν). This old triple compound particle (τοι, γαρ, ουν) is in the N.T. only here and Heb 12:1. Paul applies the logic of the case.

He that rejecteth (ο αθετων). This late verb (Polybius and LXX) is from α-θετος (α privative and verbal of τιθημ, to proscribe a thing, to annul it.

But God (αλλα τον θεον). Paul sees this clearly and modern atheists see it also. In order to justify their licentiousness they do not hesitate to set aside God.