Why was John given the vision of the saved in heaven (Rev. 15:2-4) after he was shown "seven angels with the seven last plagues" (verse 1, NIV) and before the actual beginning of the plagues (Rev. 15:5-16:1)?
The vision of Revelation 15:2-4 was not given to demonstrate that the saved will be in heaven during the pouring out of the plagues on earth. The righteous living on the earth when Jesus comes in glory "have come out of the great tribulation" (Rev. 7:14, RSV). The vision of Revelation 15:2-4 depicts those upon whom the plagues did not fall because, while on earth, they gained the victory over the beast, its image, and its mark (verse 2). These victorious ones are presented in shining contrast to unbelievers, who will suffer the plagues. "While the wicked are dying from hunger and pestilence, angels will shield the righteous and supply their wants. To him that 'walketh righteously' is the promise: 'Bread shall be given him; his waters shall be sure.'. . . Isaiah 33:15, 16, 41:17."--The Great Controversy, p. 629.
Will God send the plagues? Rev. 16:1, 5, 7, 19 (compare Exod. 7:1-5, 17; 8:2, 19, 21; 2 Peter 2:4-6, 9).
God sent the plagues on Egypt, just before He delivered His people. These plagues were a type of the final plagues God sends on the world before He delivers His people to take them to the heavenly Canaan. "God's love is represented in our day as being of such a character as would forbid His destroying the sinner. Men reason from their own low standard of right and justice. . . . God is a moral governor as well as a Father. He is the Lawgiver. He makes and executes His laws. Law that has no penalty is of no force."
"The plea may be made that a loving Father would not see His children suffering the punishment of God by fire while He had the power to relieve them. But God would, for the good of His subjects and for their safety, punish the transgressor. God does not work on the plan of man. He can do infinite justice that man has no right to do before his fellow man. Noah would have displeased God to have drowned one of the scoffers and mockers that harassed him, but God drowned the vast world. Lot would have had no right to inflict punishment on his sons-in-law, but God would do it in strict justice."--Last Day Events, pp. 240,241.