To wit . Not in the Greek. But the infinitive (εινα) clause is epexegetical and gives the content of the revelation, a common idiom in the N.T. Τα εθνη is in the accusative of general reference. Paul is fond of compounds with συν and here uses three of them.

Fellow-heirs (συνκληρονομα). Late and rare (Philo, inscriptions and papyri). See also Romans 8:17.

Fellow-members of the body (συνσωμα). First found here and only here save in later ecclesiastical writers. Preuschen argues that it is equivalent to συνδουλος in Colossians 1:7 (σωμα in sense of δουλος).

Fellow-partakers (συνμετοχα). Another late and rare word (Josephus). Only here in N.T. In one papyrus in sense of joint possessor of a house.