ὅτι. Sc. ἐδίωκεν. ὡς ἐξ ἔργων = with the idea that they could attain by starting from works.
τῷ λίθῳ τοῦ π. Allusion to Isa 8:14, LXX[198] λίθον πρόσκομμα. The sense in Isaiah is that the Lord of Hosts will be a sanctuary for Israel if they trust in Him; they will not then find Him as a stone to stumble against. The absence of faith makes Him so.
[198] LXX. the Septuagint Version of the Old Testament