Verse 18 gives us the context, the proclivity of mankind to suppress the truth. Some translations say they suppress the truth “by” (rather than “in”) their unrighteousness, reinforcing the link between disobedience and blindness (Deuteronomy 28:28-29; Psalm 111:10). God has given all humanity a measure of understanding, at least about His existence, but most turn away in favor of something false that seems more reasonable to them. Even though Satan and his demons whisper their deceptions in the background, the people blind themselves through their choices. Satan forces no one; as with Eve, he adds fuel to an already-kindled fire of carnality and resistance to the truth.
People choose to close their eyes, and as the apostle Paul writes in verse 24, God responds by giving them over to that choice. He does not approve of or accept what they do but allows it to play out. At some point, they will understand the futility of rejecting God's way.
Paul states this principle again in verse 26: God gives them up to what is in their hearts, not realizing that they have cursed themselves by their choices. Ironically, they probably feel relief and liberation, having cut themselves off from bearing any accountability to uphold God's standards.
What begins with suppressing the truth in verse 18 ends with a debased mind in verse 28. As part of His judgment, God gives people over to the dominion of their choices' consequences. These verses depict God essentially taking His hands away to allow people to mess up their lives still further. However, other scriptures show God actively amplifying sin's effects; sometimes, God makes the spiritual plight of those rebelling against Him even worse.
In Amos 8:11, God calls for a famine of hearing His Word. It is a different metaphor but still analogous to blinding in that He diminishes the understanding of the people. The famine of hearing is a devastating curse because God takes away the very thing that could help the nation: The ability to hear His truth. His action may seem cruel, but the fault does not lie with Him but with the peoples' previous choices. We have a saying today, “Be careful what you wish for.” Israel yearned after her neighbors' paganism and so did not heed the revelation He had given to her, thus He began to take away her understanding.
