These verses are points of contention for theological discussions because of the repetition of the words “all” and “many.” Evangelical groups adhering to an Arminian or semi-Pelagian point of view emphasize the “all” and Calvinistic circles emphasize the “many.” The problem with both is missing the real import and emphasis of the verses. Try reading these verses again and replace the words “all” and “many” with “a great number.”
18 Consequently, just as one trespass resulted in condemnation for a great number, so also one righteous act resulted in justification and life for a great number. 19 For just as through the disobedience of the one man a great number were made sinners, so also through the obedience of the one man a great number will be made righteous. (My revision)
All the sudden the real context of the verses comes through. The issue Paul is addressing is not the quantity of people, but the contrast of Adam and Christ. This has been Paul’s argument since verse 12 of chapter 5. Simply put, Adam brought his people into judgment, condemnation and disobedience that made us sinners, and Jesus brought his people the free gift of justification of life and righteousness.
The import is that this is what we are to gather from these verses, the concepts of sin, atonement and the representative and substitutive nature of belonging to Adam or Christ. This is novel because it is not mentioning anything about being a Jew or of Abraham. The contrast is Adam or Christ only.
Now as to the quantity of people affected by these two representative heads and who all partakes of the justification of Christ, we will have to look to other passages of the Bible. We do not have to go far because Paul makes this part of his argument within Romans, that it is those who by faith believe in Christ for their salvation that will partake of His free gift.