Hi Buzz,
In Matt 25:31-46 you're interpreting Jesus's reference to 'brethren' as refering to Christians. I think this is wrong, for two reasons.
1) Jesus refers to anyone who's doing God's will as 'brethren' (Mar 3:33-34) not just disciples or Christians. God's will is -as Jesus repeatedly said- 'Love God and love your neighbour as yourself', *not* 'believe in Jesus'.
2) As Matt 25:31-46 is about the Final Judgement, the term 'nations' would imply everyone who has ever lived, both before and after Jesus.
In this context, 'brethren' couldn't possibly refer to Christians. It makes no sense to judge people who lived B.C on how they treated Christians, as Christians weren't around then. The only plausible meaning for 'brethren' would be 'fellow men'.
As for Jesus saying that the only way to heaven is through him, this is nothing but affirming his role as the Judge, which is emphatically confirmed in John 5:22.
In short: it doesn't matter what you believe, Jesus will judge you on how you behave.
"In life, you have to face that some days you'll be the pigeon and some days you'll be the statue."
™ Version 4.2