Why can God make the promise that if He is for us no one can be against us?
God can make that promise because He did not spare His own Son but delivered Him up for us all. Since God has already given His Son for believers, Paul reasons that no opposing power can ultimately prevail against them, even if it touches the flesh.
Les teaches that Romans 8:32 gives the reason God can make such promises: He who spared not His own Son, but delivered Him up for us all. The promise that God is for us and nothing can be against us rests on that finished work. Les explains that regardless of what someone or the powers that be may do to the flesh, they can never touch that invisible eternal part of us that is in God's hands. He also stresses that Paul never writes to anyone but believers, so the pronoun "us" means Paul and his fellow believers.
He that spared not his own Son, but delivered him up for us all, how shall he not with him also freely give us all things?