Sin is a major problem that impacts many parts of life, but God does provide a solution. In the first three problems, Paul talks about how sin is a major problem for Gentiles, judgers, teachers, and Jews -- and then Paul concludes that "all have sinned and fall short of the glory of God" (3:23). I know that talking this condition doesn't feel good, but identifying problems is important in problem solving (not to mention how diagnosis is important).
Anyway, the good news talks about how "the righteousness of God apart from the law is revealed [...] through faith in Jesus Christ" (3:21-22). This righteousness is considered "apart from the law" because it's a route that does not involve keeping God's standards 100% (because we can't), though the righteousness of God is "witnessed by the Law and Prophets" (3:21), meaning that God's plan meets the righteous calls that God did (especially for the Judeans). In particular, Jesus Christ was able to do what no one else can do -- keep God's standards 100%. So Paul is essentially talking about how Jesus Christ accomplished the law on behalf of us (and some of you are familiar with how he !
The righteousness of God is available "through faith in Jesus Christ, to all and on all who believe" (3:22). Possibly due to how none of us are able to attain God's righteous on our own. But God made His righteousness available for everyone, and we receive it by faith (in particular, believing in Jesus as Lord and Savior). Believing in Jesus (AKA having faith in Jesus) is essentially accepting that we need Lord Jesus (who is Son) because of screwing up in the past and counting on Jesus and his resurrection for providing His righteousness. That's it for us. Jesus as Lord (and Son) met God's standards on behalf of us and has the righteousness, and Jesus as Savior saves us from the sad state of failing to match God's standards. When we believe in Jesus, we are guaranteed everlasting life that would ultimately not end.
For the readers who decided to accept Jesus and count on Him for saving us, you are guaranteed eternal life. I don't have a "I raise my hand" button or "I accepted Jesus" button because faith starts internally. (You can raise your hand though.)