Worshipped and served the creature (Rom 1:22)
What constitutes the backsliding Paul relates which a conscientious person would endeavor to avoid? They didn't apply their knowledge since "they did not honor Him as God, or give thanks" (Ro 1:21). It was a deliberate decision because "they did not see fit to acknowledge God" (:28). They "suppress the truth" (:18). They "exchanged the glory" (:23) and "the truth of God for a lie" (:25). In their pride they claimed "to be wise" (:22) and "worshipped and served the creature rather than the Creator" (:25). By their behavior they sealed their own fate. When they "became futile in their speculations" (:21) it gave "birth to sin . . . [which] brings forth death" (Jas 1:15). Therefore God "gave them over" (Ro 1:24, 26, 28) to the desires of their hearts. "Their foolish heart was darkened" (:21) in the form of "impurity" (:24) and "degrading passions" (:26) which are acts of sexual indecency. They received "in their own persons the due penalty of their error" (:27). Finally "God gave them over to a depraved mind" (:28) and worse things happened. This would bring into question the doctrine of eternal security. If they didn't "hold fast . . . until the end" (Heb 3: 6) then they wouldn't be saved.