The case against soundbite evangelism
The question: "Pretend that I have never heard about this Jesus before, and you have 30 seconds to give me all the information I need to be saved. What information are you going to give me? Will you have enough time to make sure the information is correct and complete?"
Here were some of the answers:
Romans 3:23 / 5:8 / 10:9 - something like that
I might use 1 Cor 15:1-4. (this was mine).
Romans 1:20 Come on now. I'm suprised at you guys.
My actual favorite verse even if I have more than 30 seconds is Isaiah 53:6. No one is exempt from doing his own thing, at least at some point in their lives, and so this verse points them to the need for redemption, and Christ as the answer.
I believe I could do it in less than 30 seconds easily: "God clearly states you & I are sinners bound for judgment, but God has sent One to reconcile us to Him. This One’s name is Jesus & He took our punishment for us through He had no sin of His own. Pray to Him & His Father in Heaven right now to save you & He promises that when you die, you will not suffer for your sins."
I'm going to argue that it can't be done. Oh, not that you can't try to do it, but just that this isn't the best way, and it will usually be improper.
Jesus didn't do it this way
He took time to assess people, and then met them at their needs. I won't go through every example, but let's look briefly at one, Luke 24. I think what Jesus did is instructive. He began "with Moses and all the Prophets," and he "interpreted to them in all the Scriptures the things concerning himself." Now the journey to Jerusalem was 7 miles, and at a walking pace that would take some time. Also, depending on how you read this, these men were some who had been followers of Jesus (given the context, I think this is correct). So if people who were actually there had to have explanations (particularly if they were also his followers and therefore had already heard his teaching), how much more so does someone today need to have the context set.
Paul didn't do it this way
Look at Romans, described by someone I know as the greatest Gospel tract ever. Paul starts with the bad news, and only when a person is convinced of their need for a savior does he tell them about him. But what if someone doesn't know the Bible? Can they really understand Paul's argument that "one man brought sin into the world" if they have never read Genesis 3? There's an underlying assumption of knowledge of the rest of Scripture.
I don't do it this way
I have almost daily evangelistic interactions in my home--with my children. I'm not taking a 30 second approach with them--trying to coerce some sort of decision; instead, I'm trying to inform them of all of Scripture's teaching, believing that God's work through the Holy Spirit will convert them.
What do we say then, to the situation above? At a minimum, I think we must try to gauge what a person knows about the Bible, and about Jesus Christ. If we reject the notion that people are justified "because they have responded to the grace of God revealed to them, apart from considerations of their success in fulfilling whatever their holy books or religious traditions prescribed for them," and instead embrace the exclusivity of the gospel, and its grounding in the historical person and work of Jesus Christ, then we must make all of this known to people for them to have a chance to accept or reject it.
Also, given the modern-day proliferation of other religions and cults, it may take some time to tease out what people believe about Jesus--there's a difference between how a Mormon or a Jehovah's Witness sees Jesus, and that difference is not peripheral, it is the key to the Gospel.
I would also note that, in a situation like Saul's, he had a large amount of knowledge about Jesus and who he claimed to be.
In the end, I would suggest that what people need to believe--"Believe on the Lord Jesus Christ and you will be saved"--is not all they need to know; rather, what they believe is informed by what they know, and that they cannot come to a belief in Jesus without knowing about him.
None of this is to say that you can't make a 30 second gospel presentation and someone be saved. But if that happens, it is most likely because others in the past have been faithful to sow and water, that God has given the growth, and placed you in a position to reap the harvest.