As far as how much someone needs to believe, I think the Romans Road does capture it well:
But what does it say? “The word is near you; it is in your mouth and in your heart,” that is, the message concerning faith that we proclaim: If you declare with your mouth, “Jesus is Lord,” and believe in your heart that God raised him from the dead, you will be saved. For it is with your heart that you believe and are justified, and it is with your mouth that you profess your faith and are saved. As Scripture says, “Anyone who believes in him will never be put to shame.” For there is no difference between Jew and Gentile—the same Lord is Lord of all and richly blesses all who call on him, for, “Everyone who calls on the name of the Lord will be saved.” (Rom 10:8-13)
As Carson says in this post expanding on the ABC presentations of the gospel, these kinds of presentations - like the Romans Road - are biblical. And, since
they can absolutely lead to salvation. For, no matter how we hear, it is Jesus himself who saves. Yet, I think it’s key to recognize that ABC presentations don’t contain the full story that’s within the Gospels and they often may not draw us in in the same way that the fuller story of the gospel does nor do they help us to see the nuances of who Jesus is or what it looks like to live out our faith in the same way.
As for the how much fruit aspect, I think @Carson’s response covered it well, I would only add that, as you noted, the thief on the cross simply asked Jesus to remember him. He died before any more fruit could be seen, yet surely his life would have borne that fruit out over time. Nevertheless, as regards discerning from our earthly perspective, we cannot see others’ hearts. Thankfully as @Carson shared,
Concerning, this
The way I see the gospel as a story fitting in with concepts like the forgiveness of sins, is that it shows us the broader context and the motivating reasons for why we ought to want our sins forgiven. The gospel as a story not only tells us that our sin separates us from God, but shows God’s steadfast love as He consistently blesses, warns, teaches and directs His people. It shows how the righteousness and holiness that He calls us toward is not arbitrary but is for our good, and the grievous consequences that come as a result of sin time and time again as His people continuously stray throughout history. Failure after failure - even from those such as Moses and David - drives home our inability to live righteously on our own and our great need for reconciliation and a new heart. In this light, the hope and joy of Jesus’s sacrificial death on the cross for us, bringing reconciliation and a means to be created anew and given God’s own Spirit is made so much the greater! For, by entering into the story, our great need for God and our total reliance on Him and His great, enduring love and forgiveness is made clear through thousands of lives and contexts in which He works. It’s not a story that ends there either, but one which presents us blameless before God and living with Him in new creation! A part of the story which helps ground our hope, showing the glorious future that we have to look forward to. And yet, a part so frequently left out that I think many would be surprised to learn that we look forward to life in the New Jerusalem in new creation with physical bodies rather than an eternity of disembodied existence in heaven.
I hope that can be a helpful perspective and I’d love to hear your thoughts! If you haven’t yet gotten to check out Carson’s post I linked expanding on his thoughts about ABC presentations, I’d definitely recommend giving it a read!