Commentary on Romans 1:18-23

The thesis St Paul announced explicitly in Romans 1:16-17 is taken up and developed in Rom 3:21-4:25 in light of what was said in the intervening material (Rom 1:18-3:20).
 

THROUGH THEIR SINS THE PAGANS HAVE LAPSED INTO IDOLATRY
A Summary of Romans 1:18-23

Read Romans 1:18-23~Having asserted that justification comes only through faith (Rom 1:1-17), the Apostle here proceeds to indicate that both Gentiles and Jews have grievously sinned, and are therefore in need of redemption (this is the dominant theme of Rom 1:18-3:20); this redemption can now be obtained through faith in Christ (the major point of 3:21-4:25).

In the present section St. Paul points out the sinfulness of the pagans. They could have known God, and did know Him, to some extent; but they failed to render Him the homage which was His due, with the result that the notion of Him which they had through human reason became obscured, and they turned in their wickedness to dumb idols.

COMMENTARY ON ROMANS 1:18-23
Fr. Callan is using his own translation. Links are to the NABRE 

 Rom 1:18. For the wrath of God is revealed from heaven against all ungodliness and injustice of those men that detain the truth of God in injustice: 

For (γάρ = gar) indicates the reason why a revelation of the “justice of God” was necessary. Some, however, think that γάρ does not here denote a strict consequence, but rather a mild opposition (Lagrange). The threefold use of γάρ (gar) in verses16, 17 and 18 establishes a close connection between the content of those verses. According to Shedd, γάρ “introduces the reason why God has revealed the δικαιοσυνη (dikaiosune = righteousness) spoken of: namely, because he had previously revealed his ὀργή (orgē = wrath). This shows that mercy is meaningless except in relation to justice, and that the attempt, in theology, to retain the doctrine of the divine love, without the doctrine of the divine wrath, is illogical.” (Text in blue my additions to the quote from Shedd).  For some reason that escapes me, the Protestant NIV Bible simply eliminates the word for (γάρ = gar), beginning the verse with The wrath of God.  James Moffatt and C.H. Dodd insist on taking γάρ as an adversative (But the wrath of God); a usage it rarely has. On cannot introduce a dichotomy between God’s Justice and his wrath, they are “two sides of the same coin” (Frank J.Matera).  Is there, perhaps, a certain embarrassment on this among some commentators? ~But if our injustice commend the justice of God, what shall we say? Is God unjust, who executeth wrath?  (I speak according to man.) God forbid! Otherwise how shall God judge this world?  For if the truth of God hath more abounded through my lie, unto his glory, why am I also yet judged as a sinner?  And not rather (as we are slandered and as some affirm that we say) let us do evil that there may come good? Whose damnation is just (Rom 3:5-8). 

The wrath of God is revealed, etc., is understood by older critics to refer to the anger which God will display at the Last Judgment. Cornely and other modern authorities understand it of anger already manifested. Doubtless it is to be understood of anger already displayed, the full and final issue of which, however, will be felt only at the Last Judgment. The Greek word αποκαλυπτεται ( = apokalyptetai, "is revealed") is a present indicative middle. In other words, it denotes action already in progress (present indicative). The wrath of God is already  being manifested. 

Wrath is attributed to God anthropomorphically, and means here nothing more than a manifestation of His justice (2 Sam 19:2; Neh 1:6). Without doubt God will at the Last Judgment manifest His justice towards all sinners in ways unseen and unrealized here below. St. Paul often speaks of God’s wrath in the eschatological sense (Rom 2:5; 5:9; 1 Thess 1:10, etc.), but it is evident from the present tense of the verb here, αποκαλυπτεται ( = apokalyptetai, "is revealed"), and from the context, that the Apostle is now speaking of wrath which God has already exercised on the Gentiles. Father Callan’s reference to the context is a reference to verses 24, 26 and 28 and the phrase “God gave them up”.
Concerning The wrath of God in Rom 1:18. The phrase indicates "God's vindictive justice in punishing sin. While this is ultimately reserved for the Last Judgment, it is visited upon men even in this life, hence (Paul says, using the present tense) it 'is revealed'" (CBA, Comm. on the N.T.). Although "vidictive" often has a pejoritive sense in modern usage, it should be remembered that it is related to "vindicate."  Here I will first encourage everyone to acquire a decent Catholic Bible Dictionary as an aid for gaining a better understanding of such difficult mysteries as "the wrath of God" (I've listed a couple suggestions at the end of this post).

Is revealed from heaven, i.e., God’s judgments on the sins of the Gentiles are sent out, so to say, from the place of His dwelling, from the seat of His presence. 

Ungodliness means impiety, as opposed to the virtue of religion, which renders to God His due. 

Injustice expresses more openly what is also implied in “ungodliness”; for to fail in piety is likewise to fail in justice to God. Both words refer to the injustice, immorality and other sins of the Gentiles.

The pagans are said to detain (κατεχοντων) the truth of God, etc., inasmuch as their state of injustice and sin excluded possession of the truth, and kept it, as it were, locked up from them. Truth and injustice are opposing forces; and as there is question here  of religious or moral truth, the former (i.e., truth) is said to be excluded, kept away, enslaved (κατεχοντων) by the latter. Concerning detain (suppress) the truth in Rom 1:18. "Wickedness is a force opposed to truth which, if unrestrained, would expand injustice" (CBA Comm. on the N.T.).

Of God is not in the Greek; hence Dei  (God) after veritatem (truth)of the Vulgate should be omitted.

Rom 1:19. Because that which is known of God is manifest in them. For God hath manifested it unto them.

In this verse St. Paul says that a natural knowledge of God, of His existence and of some of His attributes, to which unimpeded human reason can always attain, was possible to the pagans; and thence it follows that, had they rendered to God, as they could and should have known Him, the homage that was His due, they would have received further help from Him to enable them to lead moral lives and thus attain salvation. The words to το γνωστον (= gnoston, "is known") of this verse mean the objective notion or knowledge of God, which man is able to acquire from the visible universe, notitia Dei objective sumpta (= the notion or knowledge of God objectively taken); γνωστον (gnoston) is always used in this sense in the New Testament. 

Is manifest, etc., i.e., is clear to them, made manifest externally among them. The Gentiles had before them that clear knowledge of God which is possible to man through the natural light of reason operating on the visible world around him (St. Thomas). Rom 1:20. For the invisible things of him, from the creation of the world, are clearly seen, being understood by the things that are made; his eternal power also, and divinity: so that they are inexcusable.

The Apostle wisely addresses to the Gentiles first an argument from the natural order. The nature and attributes of God are called invisible things because they are not naturally perceptible as they are in themselves; but, by reason of things created and naturally visible, human reason has been able from the beginning of the world to rise to a knowledge of the existence of those things which it otherwise could not know, and which are at all times invisible to the senses (1st Vat. Conc., Sess. III. cap. 2). Ever since there was a created mind capable of reflecting on the visible universe, therefore, it has been possible for man to rise to a knowledge of the existence of a Creator.

Naturally the first attribute of the Creator, which would be suggested to man’s mind, would be that of power; and upon further reflection it would be clear that such power could reside only in divinity. Hence the Gentiles were inexcusable in not knowing the existence of some of the attributes of the one true God, and in not rendering to Him the homage which was His by right. Catechism Link for Romans 1:19-20: Ways of Coming to Know God~CCC 31-38;   

Rom 1:21. Because that, when they knew God, they have not glorified him as God, or given thanks; but became vain in their thoughts, and their foolish heart was darkened.

Because (διότι = dioti) shows the connection with the preceding verse and introduces a development of the theme therein stated. St. Paul now goes on to explain why the pagans were inexcusable. Not because they had a perfect and explicit knowledge of God, and then refused to pay Him due honor and worship; but because they could have had sufficient notion of His existence and nature not to be guilty of the ignorance with which they are here reproached. Hence St. Thomas says that the first fault of the Gentiles was one of ignorance. Had they made proper use of the first knowledge which they had of God, they would have progressed to further understanding of Him, and would have recognized Him as God; they would have worshipped His supreme majesty, and rendered to Him honor and thanks as the Master and source of all good and blessings. But, having wilfully paralyzed the first help and obscured the first light that was given them, they were plunged into deeper darkness and error, with the result that, instead of thanking God as the cause of benefits, they potius suo ingenio et virtuti suae bona sua adscribebant (St. Thomas). 

Heart here represents all of man’s higher faculties, both volitional and intellectual. 

Rom 1:22. For professing themselves to be wise, they became fools.

This verse does not explain what precedes, but rather indicates the supreme degree of error into which the pagans had fallen. The words are general and embrace not only philosophers, but all the Gentiles, represented by the most cultivated people. 

For (Vulgate, enim) is not represented in the Greek. Catechism Link for Romans 1:21-23:  Honoring God as Creator~CCC 2084-3141.

Rom 1:23. And they changed the glory of the incorruptible God into the likeness of the image of a corruptible man, and of birds, and of fourfooted beasts, and of creeping things.

So far in their perversity and ignorance did the pagans go that they paid to mere creatures, such as men, birds, beasts, and reptiles,—nay, even to the images and representations of these things, the honor and worship which is due to the eternal God alone. The folly of the Gentiles was in their conception
of the Deity, whom they came to regard as represented by created and material objects; and their false notions begot a false worship. 

The likeness of the image, i.e., the image which represented such things as man, birds, beasts and the like. Among the Greeks and Romans idols had the figure of a man, but among the Egyptians they took the form of animals.

FOR FURTHER STUDY AND DISCUSSION

The Wrath of God (Rom 1:18). I would recommend reading these two articles (1) What is the Wrath of God? by Msgr. Charles Pope. (2) This online excerpt from the Navarre Bible Commentary on Romans 1:16-25. See especially the summary it gives of verses 18-32.
Lectionary Link and Discussion Prompt: Romans 1:16-23 is part of the first reading for the Tuesday of the 28th day in Ordinary Time, Year I. It is used in conjunction with Ps 19:2-5 and Luke 11:37-41. What connections do you see?

BIBLE DICTIONARY RECOMMENDATIONS
Links to Amazon.com.

New World Dictionary To the New American Bible. The most succinct and inexpensive of the volumes listed here. I think it's out of print but used copies can still be found for sale online.

McKenzie's Dictionary of the Bible. By Fr. John McKenzie. Was (and probably still is) the standard Catholic Bible Dictionary. It is somewhat outdated. 

Catholic Bible Dictionary. Edited by Dr. Scott Hahn. The most recent Catholic Bible dictionary. 

Dictionary of Biblical Theology. By Fr. Xavier Leon-Dufour. This is somewhat different than a Bible Dictionary. The content is much less broad (350 articles compared with 2,000 + in McKenzie's), but the subjects that are treated are presented in some detail. This is for the more advanced or committed.