Is Paul saying in Romans 3 that all have the ability to be justified by grace? If so, how does this fit with the rest of his teaching about being selected before birth?
Your confusion comes from taking these two verses out of their larger context in Romans 3. When we take vs.24-25 out of context, we lose sight of what Paul meant when he used the word “all.” So by returning to the context of the passage, we find the intended meaning of the word “all."
Paul describes our righteousness may be found. The righteousness required to enter Heaven is the righteousness of God, given to us by God’s grace, apart from works. Notice in v.22 that Paul adds that this salvation is “for all those who believe; there is no distinction…” Paul is saying that the means of salvation is the same for both Jew and Gentile. There is no separate plan for Gentiles apart from Jews. We find this conclusion confirmed further when we look at the end of the passage, as Paul concludes God is the God of both Jew and Gentile.
Therefore, when Paul uses the term “all” in this passage, he is referring to both Jew and Gentile. The word “all" doesn’t refer to all humanity.
For a greater understanding of these things, we strongly recommend you listen to our entire Romans Bible study.