Was seen for many days (ōphthē epi hēmeras pleious). The common verb (first aorist passive indicative of horaō, to see) for the appearance of the Risen Christ, the one used by Paul of his own vision of Christ (1Co 15:8), which is not reported by Luke here. For more days (than a few), the language means, forty in all (Act 1:3).
Of them that came up with him (tois sunanabāsin autōi). Dative (after ōphthē) articular participle (second aorist active of sunanabainō) with associative instrumental case (autōi), the very men who knew him best and who could not be easily deceived about the reality of his resurrection. But this fact rules Paul out on this point, for he had not fellowshipped with Jesus from Galilee to Jerusalem.
Who are now his witnesses (hoitines nun eisin martureōs autou). The very point that Peter used to clinch his argument with such powerful effect (Act 2:32; Act 3:15).