The fifth angel carries the seal of the living God. The symbolism is not explained, because the seal of the living God has already been described in Scripture: the Holy Spirit is the seal with which every believer is sealed as God’s upon conversion (Eph 4:30, 2 Cor 1:22). This chapter describes conversion.
“Thus the six seals … simply tell the story of the historical periods [of the Roman Empire as outlined in a basic college textbook]. The next period of the interlude of the seventh chapter [of Revelation] indicates that what follows hard on this should be a multitude of people becoming Christians. They dip their garments in the blood of the Lamb! The next chapter … that covers the period after Constantine from 325 to 460 is called: ‘The Rise of the Christian Church.’”
Fred Miller, Revelation: A Panorama of the Gospel Age, Ch. 3, “The Opening of the Seven Seals.”