"Since the creation of the world His invisible attributes, His eternal power and divine nature, have been clearly seen" (Ro 1:20). Truth is reality being "understood through what has been made" (:20). The process of observation requires that the five senses be used. It means you can look up at the stars and understand about God. Paul phrases it as knowing about God as opposed to personally knowing God. Thinking predicates reasoning which includes deciding if something is true or false. Then with an open mind you choose by believing in the best alternative. It involves wrestling with various possibilities until the one with the highest probability of being true wins out. But when "that which is known about God is evident within them" (:19) it was God who "made it evident to them" (:19). Jesus told Pilate "'for this I have come into the world, to testify to the truth'" (Jn 18:37) for which Pilate asked, "'What is truth?'" (:38). At that level it is absolute certainty and a revelation to their conscious understanding. "With the heart man believes" (Ro 10:10). Nonetheless "they exchanged the truth of God for a lie" (1:25) and "their foolish heart was darkened" (:21).
Mankind has been given a general revelation or moral sense. They "do instinctively the things of the Law, these not having the Law to themselves" (Ro 2:14). "They show the work of the Law written in their hearts, their conscience bearing witness" (:15). However the people did not conduct themselves correctly. But "in the generations gone by He permitted all the nations to go their own ways" (Ac 14:16). "In the forbearance of God He passed over the sins previously committed" (Ro 3:25). Nevertheless they are sins. The "Gentiles also walk, in the futility of their mind being darkened in their understanding" (Eph 4:17-18) having "given themselves over to sensuality" (:19). But the message of general revelation does not spell out a plan of salvation. "Having overlooked the times of ignorance, God is now declaring that all people everywhere should repent" (Ac 17:30). He "desires all men to be saved and to come to the knowledge of the truth" (1Ti 2:4). Paul said, "I was shown mercy because I acted ignorantly in unbelief" (1:13). The Gentiles sinned "because of the ignorance that is in them because of the hardness of their heart" (Eph 4:18). But he is "patient toward you, not wishing for any to perish but for all to come to repentance" (2Pe 3:9). "Shall not the Judge of all the earth do justly?" (Ge 18:25). "'He who believes in Him is not judged; he who does not believe has been judged already" (Jn 3:18). "'The one who did not know it, and committed deeds worthy of a flogging will receive but few'" (Lk 12:48). "I take no pleasure in the death of the wicked, but rather that the wicked turn from his way and live" (Eze 33:11). "When we are judged, we are disciplined by the Lord so that we will not be condemned along with the world" (1Co 11:32).