Radiance is simply part of the glory of heavenly beings; with respect to angels, “their faces were more radiant than the radiance of the sun” (2 Enoch 19:1). The point is that Satan (in a sort of trickster role) disguises himself in impersonating others. In the contemporary exegesis of Genesis 3, Satan disguised himself as a serpent. In the Testament of Job, Satan disguised himself as a beggar (6:4), as a king of the Persians (17:2), and as a baker (23:1). He also impersonated angels: “When the angels of God went to worship, Satan came in the form of an angel and sang hymns to God as the angels” (Greek Life of Adam and Eve 7:1-2). So inasmuch as Satan poses as an angel, he would of course be radiant. But this is not an intrinsic characteristic, it is part of the disguise. Paul is telling his followers to beware of Satan as he could be where you least expect. Testament of Job which posits Satan as disguising himself to appear as something else. Paul uses the middle form of the verb μετασχηματίζω with the preposition εἰς, which often has the sense of disguising or masquerading as someone else, which he applies to Satan’s servants (διάκονοι) the false apostles (ψευδαπόστολοι) in v. 13, 15. For a contemporary use of this construction, see Philo, Legatio ad Gaium 80.5. Philo describes Caligula as wanting to be thought a god (θεὸς νομίζεσθαι), as godlessly assuming godship (τὴν ἀθεωτάτην ἐκθέωσιν), by adopting different theatrical costumes at different times (ἐν θεάτρῳ σκευὴν ἄλλοτε ἀλλοίαν ἀνελάμβανε), so he “made himself up as the Dioscuri, or again as Dionysus with ivy, thyrsus and fawn’s skin … by remodelling and recasting what was nothing but a single body into manifold forms (ἑνὸς σώματος οὐσίαν μετασχηματίζων καὶ μεταχαράττων εἰς πολυτρόπους μορφάς)”. It occurs in the Testament of Reuben 5:6 with respect to the fallen angels taking the form of men (μετεσχηματίζοντο εἰς ἀνθρώπους) in order to mate with the daughters of men. It occurs several times in the Testament of Job to refer to the different disguises Satan adopts, such as posing as the king of the Persians (μετασχηματισθεὶς εἰς βασιλέα τῶν Περσῶν) in 17:1.
So being an angel of light is not Satan’s intrinsic nature, according to Paul, it is a transient condition adopted by him to appear as something he is not (just as his servants appear to be apostles when they are not). This theme ultimately draws on an exegesis of Genesis 3 (cf. Eve being deceived by the serpent’s lies in v. 3), which is definitely pre-Christian. The Devil is associated with the serpent in 1QH 11:16-18, 1 Enoch 69:6-7, Wisdom 2:23-24 (cf. John 8:44, Romans 16:20, Revelation 12:9), and Irenaeus also shares with the GLAE the understanding that Satan used the serpent as a disguise (Adversus Haereses 4.praef.4, in which Satan uses the serpent so he wouldn’t be noticed by God). Satan’s disguise as an “angel of light” may also reflect his identification with Belial. Earlier in 2 Corinthians 6:14-15, Paul says: What fellowship can light have with darkness? What harmony is there between Christ and Belial (Βελιάρ)”. This wording associates Belial with darkness. This association is also found in Qumran. In 1QM (the War Scroll), the forces of Belial are called the “sons of darkness” and God is said to have “made Belial for the pit, angel of enmity, in darkness is his domain, his counsel is to bring about wickedness and guilt, all the spirits of his lot are angels of destruction, they walk in the laws of darkness” (13:11-12), and 4Q177 4:12 says that “the angel of his truth will rescue all the sons of light from the power of Belial”. The spelling Βελιάρ may reflect wordplay altering בליעל (“without worth”) into בליאור (without light), as the DDD suggests.
Beliar was associated with Eden in the Testament of Levi: “And He shall open the gates of paradise, and shall remove the threatening sword against Adam, and he shall give to his saints to eat from the tree of life, and the spirit of holiness shall be on them. And Beliar shall be bound by him, and he shall give power to his children to tread upon the evil spirits” (18:10-12; cf. Genesis 3:15 and Luke 10:19 on trampling snakes). Compare also 1QH 11, in which the “torrents of Belial” overflow and flood into Sheol, opening “the gates of Sheol for all the deeds of the serpent, and the doors of the put close upon the one expectant with injustice and everlasting bolts upon all the spirits of the serpent” (11:15-29), again associating the Belial with the serpent.