In verse 21 of Romans 1, it says "They knew God" and the natural man cannot discern a spiritual God. God is a spirit and we must worship him in spirit and in truth.
Consider how William Kelly [1800's] puts it (how much am I permitted to quote, with link, of the following?

) :
"Again, this verse is not, as some suppose, limited as a preface to the proof of Gentile depravity;
it is rather the thesis in brief, which is opened out in the rest of Romans 1, 2, 3, down to verse 21, which resumes the treatment of God's righteousness, and begins the details of that which we had in Romans 1:17. I understand, therefore,
that verse 18 gives first the general description of human ungodliness in every phase, and then the unrighteousness which was at that time most conspicuous in the Jews who combined with practical injustice a tenacious hold or possession of the truth:
the former demonstrated to the end of Romans 1; the latter (after the transition of Romans 2:1-16.) pursued from Romans 2:17 to Romans 3:20. Had this two-fold aspect been apprehended in the verse before us, the rendering of the Authorized Version would not have been deserted for "restraining the truth by unrighteousness,"
which is a sense framed to meet the condition of the heathen who were supposed here to be alone in the apostle's view. The same misconception wrought mischief in lowering the character both of the revelation of God's wrath from heaven, and of the truth in order to meet paganism.
Admit the universal scope of the moral description with a specific reference to those who held the truth in unrighteousness,
and the sense which results is as easy as it is all-important, the fitting introduction to the entire episode that follows till the apostle takes up his proper theme, God's righteousness revealed in the gospel.
"The apostle next proceeds to set forth
the proofs of the guilt of men [...]
"
The next ground is not the knowable but the positively known. "Because, having known God, they glorified him not as God, nor were thankful, but became vain in their reasonings, and their unintelligent heart was darkened. Professing to be wise, they became fools, and changed the glory of the incorruptible God into* a likeness of an image† of corruptible man and birds and quadrupeds and reptiles." (Ver. 21-23.)
A traditional knowledge of God is in question; and as the former regarded man with evidence from the beginning calculated and adequate to indicate a divine First Cause, so the objective knowledge of God here spoken of was the portion of man even after the flood: indeed not till after that mighty event do we hear of idolatry. But man was
unequal to the task of preserving the holy deposit; and this, because of his moral state. When they did know God, they neither glorified Him as such, nor were they thankful. This left room for vain reasonings, which again darkened the heart instead of leading it into light. It was the self-sufficiency, and so the folly, of the creature.
"...
man departed more and more from God till the flood; after it [after the flood] he gave up the knowledge of God for the worship of the creature. The race fell into ever increasing error and evil, till a partial revelation by Moses and the complete manifestation of God in Christ judged morally the heathen world, proving its declension, not progress, its insensibility to right reason, and its departure from true traditions into the degradation of idolatry."
[bold / underline mine]
https://biblehub.com/commentaries/kelly/romans/1.htm
[perhaps with caveats regarding the fuller article, as I'm only covering what is in the quote above

]