“For although they knew God, they did not honor him as God or give thanks to him, but they became futile in their thinking, and their foolish hearts were darkened.” -Romans 1:21
The word for reminds us that Paul is laying out his argument logically. That is, one thing follows another. One choice has consequences which produces another choice with consequences, and so on.
Since mankind (humanity) possessed real knowledge of God and chose to suppress the truth; they did not honor God as God. That is, they refused to give him the honor or glory due to him. There was enough of the attributes of God revealed in creation to warrant certain honor and thanksgiving, but they refused.
Because they refused, they became futile, or vain, empty, in their thinking. In other words, the thoughts of humanity became pointless; they chose their own way—religion instead of revelation. Thus, their foolish hearts (see Psalm 14:1 and 53:1) were darkened. That is, because they rejected the light (said in their hearts there is no God), they were left in darkness (without light). It was their choice to turn off the lights so to speak.
The idolatry of mankind (worship a god of one’s own making) is a denial of reality that can only logically lead to darker and darker delusions: there is no God. Man is the maker of himself. He is “the master of [his] fate… the captain of [his] soul. The gods he creates for himself tell him he can worship as he pleases, do as he pleases, marry who he pleases, be what he pleases.
Thus, the woke, trans-delusional society we witness today is the only possible result of a society that first really knew God, but refused to honor him as God or give him thanks.