Philippians 2:18

τὸ δὲ αὐτὸ. “In the same manner” (R.V.). So Matthew 27:44, τὸ δʼ αὐτὸ καὶ οἱ λῃσταὶ … ὠνείδιζον αὐτόν. “The accusative [τὸ αὐτό] defines rather the character than the object of the action” (Lightfoot).

χαίρετε καὶ συνχαίρετέ μοι. Gaudete et congratulamini mihi, Vulg. The Greek leaves us free to explain it as either imperative (as Vulg.) or indicative. If the latter is chosen, it is little else than the imperative in disguise; he assumes their joy and congratulation in order to enjoin it. Lightfoot quotes from Plutarch (Mor. p. 347 c) the χαίρετε καὶ χαίρομεν of the messenger from Marathon; the χαίρετε there is probably indicative.

He bids them share his martyr-joy, as partners of the martyr-spirit.