[Revelation 15:1-2 (NASB)]:

1 "Then I saw another sign in heaven, great and marvelous, seven angels who had seven plagues, which are the last, because in them the wrath of God is finished.

2 And I saw something like a sea of glass mixed with fire, and those who had been victorious over the beast and his image and the number of his name, standing on the sea of glass, holding harps of God."

[Expositer's Commentary On Rev 15:1-2]:

"1 This verse forms a superscription to chapters 15 and 16. The final manifestation of the wrath of God takes the form of seven angels of judgment and is called a "sign" (semeion). This is the third explicitly identified heavenly "sign" (cf. the woman and dragon at 12:1, 3). The qualifying adjective "marvelous" (thaumaston, cf. v. 3) apparently is added because John understood the seven angels to represent the completion of God's wrath, viz., the last plagues. They are awesome as well as final in character. The word teleo means to "finish," "to bring to an end," "to accomplish," "to perform" (BAG, p. 818; cf. Rev 10:7; 11:7; 15:8; 17:17; 20:3, 5, 7). While these plagues may be the finale to the whole historical panorama of God's judgments, it would be exegetically preferrable to find a connection of them with events related in Revelation itself. As has already been argued, the first reference to the eschatological judgments is found in 6:17: "For the great day of their wrath has come, and who can stand?" After the interlude of the sealing of the saints from spiritual harm (ch. 7), the seven trumpets are sounded (8:1ff.). The sixth one involves three plagues that kill a third of mankind (9:18). The third woe (11:14) includes the bowl judgments that are called the "last" plagues. From this we may conclude that the trumpets begin the eschatological wrath of God that is finished in the seven bowls.

2 As in 14:1ff., John again focuses his attention on a scene that contrasts sharply with the coming judgment, an indication of his pastoral concern. He sees before the throne the likeness of a sea of glass shot through with fire (cf. 4:6). It is a scene of worship, and its imagery is suitable for depicting the majesty and brilliance of God, which the sea of glass is reflecting in a virtual symphony of color. No further symbolic significance than this needs to be sought here. Firmly planted on (epi can also mean "beside," NIV) the sea are those who were "victorious over the beast." They are the same ones who are seen throughout Revelation as having won out over the idolatrous beasts through their faithful testimony to Christ, even to the extent of martyrdom (e.g., 2:7, 11, 26; 12:11; 21:7; cf. 3:21; 5:5). They are the 144,000, the elect of God (7:4; 14:1), the completed company of martyrs (6:11). Note the absence of "received his mark" since mention is made of the equivalent expression "the number of his name" (see comments at 13:17). Suddenly in this dazzling scene the sound of harps and singing is heard."

[Bible Knowledge Commentary On Rev 15:1-2]:

"15:1-2. With the background of the scene in heaven described in chapter 14, John then recorded more details of God's judgment. He wrote that he saw in heaven another great and marvelous sign. Seven angels—each having a plague which all together were described as the seven last plagues—were introduced as the final step in the outpouring of God's wrath on the earth. This final "sign" relates to the preceding great signs of the woman in 12:1 and the red dragon in 12:3. These seven angels should not be confused with the two groups of three angels in the preceding chapter (14:6-20) or with any other previous group of angels.
John also saw what looked like a sea of glass mixed with fire. This is probably the same sea that was described in 4:6. Beside this sea John saw the martyred dead, the same group described in 7:9-17."

[Revelation 15:3-4 (NASB)]:

3  And they *sang the song of Moses, the bond-servant of God, and the song of the Lamb, saying, "Great and marvelous are Your works, O Lord God, the Almighty; Righteous and true are Your ways, King of the nations! 

4  "Who will not fear, O Lord, and glorify Your name? For You alone are holy; For ALL THE NATIONS WILL COME AND WORSHIP BEFORE YOU, FOR YOUR RIGHTEOUS ACTS HAVE BEEN REVEALED." 

[Expositor's Bible Commentary On Rev 15:3-4]:

"3-4 The song sung by the redeemed is the "song of Moses, the servant of God and the song of the Lamb" - a single song as vv. 3-4 show. The Song of Moses is in Exodus 15:1-18. It celebrates the victory of the Lord in the defeat of the Egyptians at the Red Sea. In the ancient synagogue it was sung in the afternoon service each Sabbath to celebrate God's sovereign rule over the universe, of which the redemption from Egypt reminds the Jew (Joseph Hertz, The Authorized Daily Prayer Book, rev. ed. [New York: Block, 1948], p. 100). Such is the emphasis in the liturgical collection of psalms and prophets John quotes from (e.g., "King of the ages"). As the deliverance from Egypt, with its divine plagues of judgment on Israel's enemies, became for the Jew a signpost of God's just rule over the world, so God's eschatological judgment and the deliverance of the followers of the Lamb bring forth from the victors over the beast exuberant songs of praise to God for his righteous acts in history.

Each line in vv. 3-4 picks up phrases from the Psalms and Prophets. Compare the following OT words with vv. 3-4: "Then Moses and the Israelites sang this song" (Exod 15:1); "your works are wonderful (Ps 139:14); LORD God Almighty (Amos 4:13); all his ways are just. A faithful God... upright and just is he" (Deut 32:4); "who shall not revere you, O King of the nations" (Jer 10:7); "they will bring glory to your name" (Ps 86:9), etc. John may or may not have heard the victors over the beast singing these actual words. But it was revealed to him that they were praising God for his mighty deliverance and judgment on their enemies. His rendering of the song may be drawn from the liturgy of the synagogue and no doubt from the early Christian church. In fact, it is precisely in connection with the ancient Easter liturgy that the church's dependence on the synagogue Passover liturgy is most easily recognizable (Massey H. Shepherd, Jr., The Paschal Liturgy and the Apocalypse [Richmond: John Knox, 1960], p. 96). The Exodus background is quite obvious throughout both chapters 15 and 16 (cf. 8:7ff. and see comments at 1:10). On the possible theme of resurrection in the hymn, see Notes."

[Bible Knowledge Commentary On Rev 15:3-4]:

"15:3-4. The victorious saints sang with harps the song of Moses... and the song of the Lamb. These may be two separate songs, the first referring to God's faithfulness to Israel and the second referring to their present situation in the Great Tribulation. Some, like Walter Scott, refer the song of Moses to Exodus 15 where Israel triumphed over the Egyptians (Exposition of Revelation, p. 315). Others, such as J.B. Smith, suggest that this is the song of Deuteronomy 32, which gives a comprehensive review of God's faithfulness to Israel (A Revelation of Jesus Christ, pp. 224-25). In this song in Revelation 15:3-4 God is praised for His great deeds, justice, truth (cf. 16:7), glory, and holiness (see 4:8 for a chart of the 14 doxologies in Rev.). Then a prediction is made that all the nations will worship God.

This description of praise to God and prediction of universal worship is in keeping with many other Scriptures and relates, of course, to the second coming of Christ and worship of God by the entire world in the millennial kingdom (Pss. 2:8-9; 24:1-10; 66:1-4; 72:8-11; 86:9; Isa. 2:2-4; 9:6-7; 66:18-23; Jer. 10:7; Dan. 7:14; Zeph. 2:11; Zech. 14:9). The awful hour of wickedness and blasphemy against God, which will characterize the period leading up to the Second Coming, will be followed by a full vindication of God's judgment and holiness in the next period."

[Revelation 15:5-8 (NASB)]:

5  After these things I looked, and the temple of the tabernacle of testimony in heaven was opened, 

6  and the seven angels who had the seven plagues came out of the temple, clothed in linen, clean and bright, and girded around their chests with golden sashes.

7  Then one of the four living creatures gave to the seven angels seven golden bowls full of the wrath of God, who lives forever and ever. 

8  And the temple was filled with smoke from the glory of God and from His power; and no one was able to enter the temple until the seven plagues of the seven angels were finished.

[Expositor's Bible Commentary On Rev 15:5-8]:

"5-8 A second and still more impressive scene follows. The door to the temple in heaven is again opened (cf. 11:19), and the seven angels dressed in white and gold come out of the temple. In a dignified manner, one of the living creatures gives a bowl to each of the seven messengers. The bowls (phiale) are the vessels used in the temple ministry especially for offerings and incense (5:8). Phiale translates the Hebrew mizraq (a bowl for throwing liquids) in most instances in the LXX. This might have been a large banquet bowl for wine (Amos 6:6), but more often it was a ritual bowl used for collecting the blood of the sacrifices (Exod 27:3). Golden bowls seem to be always associated with the temple (e.g., 1 Kings 7:50; 2 Kings 12:13; 25:15). Phiale in the Greco-Roman world was a broad, flat bowl or saucer used ritually for drinking or for pouring libations (LSJ, p. 1930).
The "smoke" that filled the temple refers to the shekinah cloud first associated with the tabernacle and then with the temple. It symbolizes God's special presence and that He is the source of the judgments (Exod 40:34ff.; 1 Kings 8:10-11; Ezek 11:23; 44:4). His awesome presence in the temple until the plagues are finished (16:17) prohibits even angels from entering it (cf. Isa 6:4; Hab 2:20).

Notes
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3 K. Boronicz argues that according to Jewish tradition the doctrine of resurrection is implicitly contained in the Law and is exemplified by the Canticle of Moses (Exod 15:1-18). Revelation 15:3-4 has a prophetic and messianic sense and points to resurrection. In their prophetic symbolism, the Song of Moses and the Song of the Lamb are identical ("Canticum Moysi et agni"-Apoc. 15:3, Ruch Biblit, 17 [1964], 81-87). Could this also be the reason why all the early church liturgies included the Song of Moses somewhere in the Easter commemoration and some also included it on other Sundays (Eric Werner, The Sacred Bridge: Liturgical Parallels in Synagogue and Early Church [New York Schocken Books, 1970], p. 142)?

In the ancient synagogue, the Haftorah (prophetic reading) accompanying the Seder on Exodus 15:1ff. was Isaiah 26:1: "In that day this song will be sung in the land of Judah We have a strong city; God makes salvation its walls and ramparts"; and Isaiah 65:24 "Before they call I will answer; while they are still speaking I will hear." Both prophetic portions are part of the texts called "Consolation of Israel" and emphasize the strengthening of the faith of Israel (Jacob Mann, The Bible as Read and Preached in the Old Synagogue, 2 vols. [New York: Ktav, 1971], 1:431-32). The Song of Moses was apparently not so frequently used in the synagogue but principally in the temple services (Werner, Sacred Bridge, p. 141).

6 There is good MS evidence for reading that angels were dressed in λίθον (lithon, "stone") instead of λίνον (linon, "linen"). But the sense of "stone" is strained and thus both Metzger (Bruce M. Metzger, A Textual Commentary on the Greek New Testament [New York: UBS, 1971], p. 756) and Swete argue for linon as the preferred reading."
 
[Bible Knowledge Commentary On Rev 15:5-8]:

"15:5-8. As John continued to look at the heavenly vision, he saw the temple described as the tabernacle of Testimony. The allusion to a temple in heaven seems to be the heavenly counterpart of the earthly temple. As it was opened, the seven angels with their plagues exited from it. The clean, shining linen of the angels indicates their purity, and the golden sashes around their chests point to the glory of God.

John saw one of the four living creatures give the seven golden bowls filled with the wrath of God to the seven angels. When this was done, smoke filled the temple, making it impossible for anyone to enter it until the seven plagues were poured out on the earth (cf. Ex. 40:34-35). Taken as a whole, Revelation 15:5-8 presents a fearful picture of impending divine judgment on a wicked world. The judgments which are to be poured out (chap. 16) fully justify this ominous introduction."