Firstly the issue of humans as created beings; the Bible describes people more in terms of your second idea; that we have all been given a rational will by God that allows us the freedom to choose between accepting God as he presents himself to us, or rejecting him. This is ultimately what ‘sin’ is all about, not our individual acts of moral wrong but an overall rejection of God’s rule over us as his created people. We are therefore each responsible for the choice we make. The Bible also tells us though, that none of us are able to fully accept God’s authority, even Christians! So in his love for his people God sent Jesus to deal with the problem of sin, once for all, as you noted in your comments about the crucifixion. The Bible puts it this way in John’s gospel “For God so loved the world, that he gave his only Son, that whoever believes in him should not perish but have eternal life.”
(Chapter 3, verse 16).
The next issue you raised was why Jesus left the job unfinished as we still need the forgiveness his death brings in the 21st century. The key point here is that the job is not unfinished. As you already mentioned Jesus’ death was once for all, the Bible says this in a letter by the Apostle Peter “For Christ also suffered once for sins, the righteous for the unrighteous, that he might bring us to God.” (1 Peter 3:18). Jesus’ death is just as valid for us today as the means of God’s forgiveness. You are correct to note though that people are cynical and even hostile to this message today, but they were just as much so in the time following Jesus’ death. But God has not changed his mind about how we find eternal life and forgiveness of our sins post-Jesus. John records these words of Jesus; “Whoever believes in the Son has eternal life; whoever does not obey the Son shall not see life, but the wrath of God remains on him.” (John 3:36) This is still the message of Christianity today, nothing has changed.
It is because of this unchanging message that Jesus himself (not modern day Christians) made the claim that he is the only way to God. How then do we decide between all the messages of religion today? We need an independent and objective testimony of what God himself says on the matter. That is why we need the Bible, it is an external source of information about God open to investigation by any one. Unfortunately the God that Dr Spong proposes is his own creation, a personal god, who is not based on evidence able to be examined by others.
I know my answers may have raised more questions for you, particularly about the reliability of the Bible but that is good! I’d encourage you to follow those up on the website (there’s a question on “Free Will” and “Will Jesus return to forgive us again?”. Or check out the things Jesus had to say for yourself by reading one of the Gospels in a modern language translation like the New International Version. Mark or Luke are a good place to start.