"When God intends to bring someone to faith in Christ, he cannot fail, although for his own reasons he may choose to wrestle long with a person before achieving that purpose." - Frame, ST, p145
He goes on to say in a footnote -
"There are also situations in which people who appear to be elect turn away from God and prove themselves not to be among his people. There are also cases in which God chooses someone without the intention of giving him the full benefits of salvation. Judas is one example (John 6:70), as is national Israel, which, because of unbelief, loses its special status as God's elect nation."