The apostle Paul uses the phrase “the many” (Gk. “oi polloi”) in a two-fold sense, but with a single meaning, that being, a certain group of people.
When used in reference with Adam, it indicates all of Adam’s descendants or the whole human race inclusively, except for Jesus.
In Romans 5:15, the phrase “the many” died because of Adam’s sin refers to the “all men” of Rom. 5:12. In other words, all human beings inclusively, 1 Cor. 15:21-22, but who are only human; therefore, Jesus is exempted because He is both human and Divine, Col. 2:9. In Romans 5:19, the phrase “the many” were made/constituted sinners refers to the “all men” (the whole human race) condemned in Rom. 5:18. So, the phrase “the many” when used with Adam refers to the group of people comprising the totality of humanity minus Jesus, while the phrase “the many” when used in reference with Christ, indicates all those who belong to Christ (those who by God’s sovereign grace were to place their trust in Christ). It’s the group of people comprising God’s chosen/elect (i.e., believers). In Romans 5:15, the grace of God and the gift (of righteousness, Rom. 5:17) by the grace of the one Man, Jesus Christ abounded to “the many”. This refers to the “all men” (all who belong to Christ; all God’s elect/chosen) justified in Rom. 5:18, and to “the many” (all God’s elect/chosen who will be) made righteous, in Rom. 5:19, through Jesus’ one act of righteousness (Rom. 5:18) or obedience (Rom. 5:19), namely His crucifixion (Rom. 3:24-25; 5:8-10).
Justification to life (Rom. 5:18) to “all men” does not mean to all of humanity inclusively, as a result of Jesus’ act of righteousness (His crucifixion, Rom. 5:8-9) because Jesus died only for the sins of His elect/chosen/predestined or sheep/saints/believers (Jn. 10:11, 15, 23; Eph. 5:25; 1:1, 4-7) and because the Bible teaches that not everyone is going to be justified to eternal life in heaven (Matt. 25:31-33, 46; 2 Thes. 1:8-9; Rev. 20:11-15), but only God’s foreknown, predestined, called believers (Rom. 8:29-30; 3:24-26; 5:1) who are Christ’s (1 Cor. 15:22-23) – New Testament Commentary – Romans by Dr. W. Hendriksen, pages 181-183; Commentary on the Whole Bible by Drs. Jamieson, Fausset, and Brown, page 1152; and The New International Commentary on the New Testament – The Epistle to the Romans by Dr. J. Murray, page 203.
Paul stated in 1 Corinthians 10:33 that he sought the profit of “the many” that they may be saved. And who are “the many”? Well, in 2 Timothy 2:10, Paul says that he endured all things for the sake of those who are “chosen” that they also may obtain the salvation which is in Christ Jesus. Or, as he says in Titus 1:1-2, “Paul … an apostle of Jesus Christ, for the faith of those ‘chosen’ of God … in the hope of eternal life…” So, “the many” who Paul sought to save were God’s “chosen” ones.
The phrase “all men” does not always mean “everyone inclusively”, as is clear from 1 Cor. 9:22 and 10:33 because Paul couldn’t and didn’t please everybody inclusively in all things inclusively (e.g., Gal. 1:10) any more than we could. He did please “all kinds” of people (Jew, Gentile, Christian, 1 Cor. 10:32) in “all kinds” of different things or ways (1 Cor. 9:19-22; Rom. 15:2). So, “all” does not always mean “everyone inclusively” in the human race, but rather can mean “all kinds of” or “all of a certain kind”, like all of God’s elect/chosen/predestined/sheep or saints/believers/church. In the same way, “the many” doesn’t always refer to all of the human race inclusively, but can and does refer to all of a certain kind of people/group/race (e.g., God’s foreknown, predestined, called, elect, chosen race, Rom. 8:29-30; 1 Pet. 1:1-2 with 1 Pet. 2:9) as it does in the last half of Rom. 5:15 and 5:19; 1 Cor. 10:33; and Isa. 53:11. Even Isaiah 53:12; Matthew 20:28; 26:28; Mark 14:24; 10:45 show that Jesus shed His blood as a ransom for many (not all) people. In these verses, quantity is spoken of (“many”) as opposed to “kind” of people (“the many”). So, the phrase “the many” can be used of different groups of people.