This is another case where the grammatical structure is not complete. It may well be taken, 'fear ... lest he spare not thee;' the beginning of ver. 21 adding a supplementary thought, of which the apostle's mind was full; still it is a broken phrase.
The subjunctive, 'if thou shouldest abide,' or 'abidest.' There are three degrees of condition in Greek: indicative, if the fact arrives; subjunctive, doubtful if it will; and conditional, of uncertain probability.
This means that the Jews would not believe in the mercy shown to the Gentiles, and thus lost the glad tidings of the grace of God for themselves; and thus, their right to the promises being gone, they come in at the end as objects of mere mercy, as any poor Gentile might be, though, by that mercy, God accomplishes his promises, to which, as to their present responsibility, they had lost all title. It is this which gives rise to the apostle's expressions of admiration as to the wisdom of God.
Or 'O depth of God's riches, and wisdom, and knowledge.'