Answer: To avoid the "distortion" of St. Paul's writings that St. Peter warns us was common even in the apostolic age (2Pt.3:15-16), we always have to read them in context. Paul's reference to predestination is a classic example of this.
When St. Paul talks in Romans Chapter 8 about "those (God) foreknew, He also predestined to be conformed to the image of His Son...And those He predestined, He also called,...(then) justified,...(then) glorified"(Rm.8:29-30), this passage seems to suggest --- if read very literally and taken out of context --- that those who go to heaven (i.e., “justified” and “glorified”) never even had to choose to believe in any god at all, much less in Jesus Christ. Instead, they were simply among the "elect" (Rm.8:33; 1Pt.1:2) who won the ultimate, eternal lottery! (Heaven picture by Daniel Holeman)
Of course, this interpretation would negate everything that St. Paul, the gospels, and all of the NT books have to say about the need to accept God's free gift of justification and ultimate glorification through what St. Paul calls the "obedience of faith" (Rm.1:5;16:26). It would also negate Jesus’ command: "Repent, and believe in the gospel"(Mk.1:15). Why "repent"? Why "believe in the gospel"? Why bother to "follow the path of faith that our Father Abraham walked"(Rm4:12), or that by others listed with Abraham in the famous "Faith of the Ancients" passage (Heb.11:1-38)? Why bother to "endure to the end" as Jesus, St. John, St. Peter, and St. Paul exhorted in the dozens of passages in scripture (some cited below*), if your salvation is not only assured once you're saved, but is assured beforehand by predestination regardless of whether you choose to believe, or choose to live an immoral life (Rm.1:18-32)?
Obviously, St. Paul is not contradicting everything he has said up to that point in Romans, nor is he contradicting what Christ or the evangelists have said or written. Yet, that is what those who embrace an overly literal interpretation of this passage, taken out of context, would have us believe.
So, what is St. Paul saying then? What he is saying (as is St. Peter in 1Pt.1:2) is that God has "foreknowledge" of everything that ever has been and ever will be. God has developed a perfect plan out of perfect love using His absolute power to accomplish it. All things have already occurred to Him, are present tense to Him, Who is "I AM". There are no surprises for God. He has taken all things, all of our free will choices, into account in exercising His absolute sovereign power. That is what St. Paul means by “predestined”. It is God’s absolute sovereign power, perfect love and provision for us, and total foreknowledge of all things, that St. Paul is trying to underscore in this “predestination passage”, which must be seen as flowing from the start of and throughout Romans Chapters 8 through 11--- “There is no condemnation for those who are in Christ Jesus”; and we can remain in Him, enduring all sufferings, precisely because we know that God has chosen us, we are His “elect”, just as were the Israelites before us (Rm.11:1-7), for whom He has sacrificed His Son, and so will provide --- and by His foreknowledge and plan from before time began, has already provided --- everything that we need to “conquer overwhelmingly through Him who loved us.”(Rm.8:37).
However, just as "God must be true even if every person is a liar" (Rm.3:4), because God's absolute free will cannot violate His absolute commitment to the Truth, Himself (Jn.14:6); so also, God's absolute sovereign power (His control over and foreknowledge of all things) cannot violate His gift of free will (part of His image and likeness imparted) to mankind: "For the gifts and calling of God are irrevocable" (Rm.11:29).
So, there you have the paradoxical mystery of "predestination": On the one hand, God clearly foreknows and calls, justifies and glorifies His "elect", His "chosen people", from before time began, providing them with overwhelming blessings: “theirs the adoption, the glory, the covenants, the Law, the worship, the promises, the patriarchs, …the Messiah”(Rm.9:4-5); ours the sacred Scriptures, Tradition, Magisterium, Catholic (universal) Church, Saints, Sacraments, including the Eucharist ---God’s perfect provision for His “predestined”, His “elect”. However, as St. Paul describes in Romans Chapters 9-11, the Israelites (God’s "chosen people", or "elect" in the OT) demonstrated that just because God has chosen you, that doesn't mean that you will choose God. God honored their free will choice or election, as He honors ours, to "repent and believe in the gospel"... or not. Since St. Paul then drives home this point in Romans chapters 9-11, again this underscores the importance of reading St. Paul's "predestination" passage in Rm.8:29-30 --- and other "hard to understand" passages, as St. Peter describes them --- in their full context.
Finally, the context for understanding this predestination passage in Romans Chapter 8 also includes the immediately preceding passages in Romans Chapter 7. There, the speaker, St. Paul as the universal man/believer, finally collapses out of exhaustion and despair, defeated in the “war” between sin (in his flesh) and his knowledge of what’s right (in his mind): “Miserable one that I am! Who will deliver me from this mortal body?”(Rm.7:24). St. Paul’s point is that he(we) is(are) defeated, hopeless in this spiritual warfare when man is self-reliant. (Note that St. Paul uses first person pronouns 50 times --- the “absolute I” --- in Romans Chapter 7). This then sets up the contrasting point in Romans Chapter 8 that those who rely upon or have faith in God through Jesus Christ, will be “more than conquerors” through the power of the Holy Spirit. We, Catholics, have an all-powerful, all-foreknowing, all-loving God on our side, who has “predestined” (perfectly preplanned and prepared,Rm.9:23) our victory through Him, if only we freely choose to accept His free gift of salvation on His terms, which St. Paul repeatedly calls the “obedience of faith”(Rm.1:5;16:26).
***”Whoever endures to the end will be saved”, Mt10:22;24:13;25:45; 1Jn2:22-28;3:6;4:11-15) (Paul -obey/endure to end:Rm1:5;2:6;3:5-8;4:12;5:1-10;6:1-6,15-19;7:4;8:13,15,18,24,-25,37;16:26; 1Cor1:8,18;4:4;5:11;6:9;7:19;8:11; 9:23-27;10:1-13; 11:29;13:2;15:2; 2Cor2:15; 5:10,15-17;6:1;7:15;10:6;11:2-4;13:5; Gal.1:6;2:19;4:9-19;5:1-9,13-21;6:7-9; Eph2:8-10;4:1-3,15-17,24;5:5-6,15;6:8; Phil.1:6,21,29;2:8,12; 3:10; Col.1:23-27,29;3:1-6,24-25; 1Thess4:3-8;5:8;1Tm1:19 2:15;4:16;5:8;6:10,19; 2Tm2:12;4:3-7; Titus1:16; Heb3:24;4:6-11;5:9;10:26-29,31-39;11:1-38;12:1,7-14 )