Paul suddenly blesses God because
by introducing a common Jewish doxology, he wants to emphasize the folly of exchanging worship of God for idols; or
he is repulsed by idolatry and offers the doxology to cleanse his mind.
Paul introduces the doxology because it highlights the folly of idol worship. He has just explained that God has made his divinity and power known through what he has made (Romans 1:20). Next, he says that instead of worshipping God as they ought, the ungodly worship created idols, and exchange the truth about God for a lie, before exclaiming that God is blessed forever! Amen.
The doxology highlights the folly of idol worship. We can detect that this is Paul’s meaning because he uses the doxology in other passages to the same effect (Romans 9:5; 2 Corinthians 11:31).
Some contend that the reason Paul introduces the doxology is that he is repulsed by the thought of the idol worship of the ungodly and needs to cleanse his mind. The notion is that articulating the folly of the ungodly causes Paul to reflect on their actions, and by doing this, his own holiness is threatened. In order to cleanse himself from the thought of wickedness, Paul articulates the doxology.
The problem with this view is that Paul often reflects on the behaviour of the ungodly but he rarely articulates a doxology. Further, he articulates a doxology on two other occasions and in neither instance is he concerned with the behaviour of the ungodly. Finally, even if Paul was repulsed by the thought of the ungodly and wished to cleanse his mind, he could simply utter a prayer in his heart.
Thus, Paul likely introduces the doxology in order to highlight the folly of idolatry.
Interpretation 1:
Paul introduces a common Jewish doxology to emphasize the folly of exchanging worship of God for idols.
Summary:
Paul explains that although God made himself known, the ungodly knowingly worship idols instead of God the Creator. In order to emphasize the folly of such an exchange, Paul includes a common Jewish doxology, blessing God.
We owe not only our existence to God but also our rational capacity. That God created us to acknowledge his power and deity is significant, and we should be ashamed when we worship created things. For God, who is blessed forever, is incomprehensibly beyond created things.
Advocates:
James Dunn
Douglas Moo
Minor differences:
Our authors agree that Paul introduces the doxology to emphasize the folly of exchanging worship of God for a created idol.
For James Dunn, Paul uses the blessing to distance himself from idol worship by indicating that all blessedness and blessing come from God alone, not created idols.1 God is a living God who is blessed forever, and it is folly to worship created idols.
Douglas Moo is less sure of why Paul includes the doxology but suggests it may be to underline the folly of worshipping idols instead of the Creator.2
Arguments
Possible weaknesses
Interpretation 2:
Paul is repulsed by idolatry and offers the doxology to cleanse his mind.
Summary:
After describing the foolishness of the ungodly who reject God for idols, Paul is repulsed. In order to cleanse his mind from the filth he has described, he articulates a doxology that refreshes his mind and reorients it toward God.
Advocates:
John MacArthur
Frank Thielman
Minor differences:
Our authors agree that Paul offers the doxology because he is repulsed by the behaviour of the ungodly.
As John MacArthur sees it, after discussing the vile behaviour of the ungodly, Paul needs to come up for air, which he does by offering a common Jewish doxology.8
Frank Thielman has a similar idea in mind, suggesting that the thought of idolatry repulsed Paul such that he could not help but bless God.9
Arguments
Possible weaknesses
25 because they exchanged the truth about God for a lie and worshiped and served the creature rather than the Creator, who is blessed forever! Amen.