Revelation 15:2 - Matthew Poole's English Annotations on the Holy Bible

This sea of glass (as our learned More thinks) hath either an allusion to the sea of glass like unto crystal, reflected upon by the lamps of fire burning before the throne, Revelation 4:5,6, or to the waters of the Red Sea congealed (while the Israelites passed over) reflected upon by the pillar of fire. Others make it the church gathered out of all nations, said to be of glass, because of its splendour and glory. Others make it to signify the world, which is said to be of glass, to let us understand God seeth through it. It seemeth to me to signify heaven; for it is mentioned as the place of the glorified saints, who had overcome all temptations to idolatry, either from pagans, (which were the beast), or from antichrist and his party, (which are called the image of the beast), and had avoided all kind of compliance with them in profession of the religion which he would endeavour to impose upon them. Harps of God signify either the most excellent harps, or holy harps. Mr. Forbes saith well, they signify hearts tempered with joy, and love, and a grateful sense of the mercies of God towards them.