and joint-heirs with Christ - as the “First-born among many brethren” (Rom 8:29), and as “Heir of all things” (Heb 1:2).
if so be that we suffer - “provided we be suffering with Him.”
that we may be also glorified together - with Him. This necessity of conformity to Christ in suffering in order to participate in His glory, is taught alike by Christ Himself and by His apostles (Joh 12:24-26; Mat 16:24, Mat 16:25; 2Ti 2:12).