Jethro,What he's saying is just because the Jews blew it doesn't mean God's plan can't be fulfilled. In context we see that what he is saying is the plan is not based on human performance, but on faith. He is in no way suggesting people who don't have faith are saved despite their failure to believe in Christ.
Surely you can see in the whole passage that there is a distinct difference between being faithless and disowning God. We know that because the two have two different outcomes. Being faithless--God is still faithful. Disowning God--he disowns you.
So, it's impossible that being faithless and denying God are the same thing that they should both have the same outcome of God being faithful despite those things. Read the whole passage. As long as you belong to him he will not/ cannot deny himself (you, a member of his own body). But if you are removed from the body through a denial of Christ he will deny you who are no longer a part of his body.
This standing in grace has a condition attached:
"1 Now I make known to you, brethren, the gospel which I preached to you, ...in which also you stand, 2 ...if you hold fast the word which I preached to you..." (1 Corinthians 15: NASB)
"...you stand by your faith. Do not be conceited , but fear; 21 for if God did not spare the natural branches, He will not spare you,either. 22 Behold then the kindness and severity of God; to those who fell, severity, but to you, God's kindness, if you continue in His kindness; otherwise you also will be cut off." (Romans 11: NASB)
It's not a condition for the satisfactory performance of work, but of continuing in the kindness of God's forgiveness. I want Kidron to show us how this is equivalent to trying to be justified by works of the law such that it can be claimed that even this continuing in God's kindness is actually a damnable work of the law that can not save.
If someone stopped believing or did not continue in His kindness, who is to blame?