Well this happened to me recently on a reframe discussion. I'd thrown in my 2 cents worth to a discussion about atonement that I thought substitution (penal substitution to be exact) was a pretty crappy analogy for atonement. Which was a just a rehash of my It's time to Substitute Substitution post and New Analogy for Atonement post.
After a little discussion someone suggested that hypothetically we could be forgiven through some other means other than Jesus death, one member was a little distressed at this suggestion. And asked
You (another member not me) wrote, 'God could mercifully forgive us whether or not Jesus came'. How? It feels like you guys are discussing the cross as some sort of symbol rather than something that achieved an ontological reality. Should I be concerned?
Regarding your question about 'God could mercifully forgive us whether or not Jesus came'. I guess I'd say because God has forgiven us (humanity) before Jesus died. (See Gen 18:26, Exo 32:32, Num 14:20, 2Ch7:14, Neh 9:17, 2Ch 30:18, Isa 55:7, Psa 65:3, Psa103:12, Jer 31:34, Jer 33:8, Eze 16:63, Mat 9:6/Mar2:7/Luk 5:21 for a start, there are more but I was being lazy) When you ask How? I'm not to sure of the mechanics. None the less God has said that he has done it. I know that this doesn't sit well with the traditional penal substitution thing and I have heard people say that God was forgiving in the old testamant with Jesus death in the future in mind but just wasn't letting us in on that yet. I'm not sure if I buy that.
Dennis Bratcher (I have no idea who he is but I like his site) would say "no" see question 5 of his 10 questions article.
So what about verses like...
Matthew 20:28 the Son of Man came not to be served but to serve, and to give his life a ransom for many.
Galations 2:20 Jesus gave himself for me.
1 Corinthians 15:3 Christ died for our sins in accordance with the scriptures.
Well this is part 1 of my little journey to try and answer this question. For more details on how my brain will process this have a look at my Liberal Evangelical post. I thought it my be fun to honestly journey with this question rather than just post an answer that I have already thought through. So here goes...