Ruler during Time of Companions of the cave.Surah Kahf.

 Ref 1. English tafsir given by Saudi Abarian embassy in India. : 

THE HOLY QUR-AN

English translation of the meanings and Commentary

Revised & Edited By THE PRESIDENCY OF ISLAMIC RESEARCHES, IFTA, CALL AND GUIDANCE

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Ref 2. Commentary on Surah Kahf

Ayah 10.  - commentary S.no. 2237. 

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Via Gemini:

Comparing the religious beliefs and policies of Emperor Decius (reigned 249–251 CE) and Emperor Theodosius II (reigned 408–450 CE) illustrates the dramatic shift of the Roman Empire from traditional Roman paganism to state-enforced Christianity.

Emperor Decius: Defense of Traditional Roman Polytheism

• Core Religious Beliefs: Decius was a committed Roman traditionalist who believed that the Roman Empire’s third-century political, military, and economic crises were caused by the loss of pax deorum (the peace/favor of the traditional gods). He believed the empire could only recover if all citizens returned to worshipping the traditional Roman pantheon.
• Imperial Religious Policy: In 250 CE, Decius issued an imperial decree requiring every inhabitant of the empire to sacrifice to the traditional Roman gods and the genius of the emperor in front of local magistrate commissions. Those who complied received a signed certificate (libellus).
• Approach to Disagreeing Faiths: Decius did not outlaw Christianity specifically by name, but his universal sacrifice mandate targeted anyone whose religion forbade polytheistic acts. Because Christians refused to sacrifice to pagan deities, his decree led to the Decian persecution—the first systematic, empire-wide persecution of Christians in Roman history.

Emperor Theodosius II: Codification of Imperial Christianity

• Core Religious Beliefs: Reigning nearly two centuries after Decius, Theodosius II was a devout Nicene (Catholic/Orthodox) Christian who viewed the emperor's primary responsibility as preserving religious orthodoxy and protecting the Christian Church. He believed imperial prosperity depended on theological unity under Christian dogma.
• Imperial Religious Policy: Theodosius II consolidated Christian rule through the Theodosian Code (438 CE), a landmark legal collection that systematically elevated Christianity while curtailing non-Christian religious practices. He actively funded churches, protected monasteries, and convened major ecumenical councils (such as the Council of Ephesus in 431 CE) to resolve theological disputes regarding Christology.
• Approach to Disagreeing Faiths: In stark contrast to Decius, Theodosius II targeted pagans, heretics, and non-conforming religious groups. His laws outlawed pagan sacrifices, stripped remaining pagan temples of their state protections, banned pagans from holding public office, and criminalized Christian heresies (such as Nestorianism).

Key Takeaway

Decius used imperial power to force monotheistic Christians to participate in polytheistic Roman rites to win back the favor of ancient gods. In contrast, Theodosius II used imperial authority to dismantle paganism and suppress non-orthodox Christian beliefs, cementing Nicene Christianity as the exclusive, legally enforced foundation of the Roman state.