From stumbling (aptaistous). Verbal from ptaiō, to stumble (Jam 3:2; 2Pe 1:10), sure-footed as of a horse that does not stumble (Xenophon), and so of a good man (Epictetus, Marcus Antoninus).
Before the presence of his glory (katenōpion tēs doxēs autou). Late compound preposition (kata, en, ōps), right down before the eye of his glory as in Eph 1:4. Cf. Mat 25:31-33; Col 1:22, where Paul has parastēsai like stēsai here (first aorist active infinitive) and also amōmous as here, but amōmētos in 2Pe 3:14.