Romans 8:17

In these words the apostle describes a fourth illustrious branch of the happiness of believers, namely, a title to the future glory. This is fitly annexed to our sonship; for as the adoption of sons entitles us to that glory, so the disposition of sons fits and prepares us for it. If children, then heirs, Rom 8:17. In earthly inheritances this rule does not hold, only the first-born are heirs; but the church is a church of first-born, for they are all heirs. Heaven is an inheritance that all the saints are heirs to. They do not come to it as purchasers by any merit or procurement of their own; but as heirs, purely by the act of God; for God makes heirs. The saints are heirs though in this world they are heirs under age; see Gal 4:1, Gal 4:2. Their present state is a state of education and preparation for the inheritance. How comfortable should this be to all the children of God, how little soever they have in possession, that, being heirs, they have enough in reversion! But the honour and happiness of an heir lie in the value and worth of that which he is heir to: we read of those that inherit the wind; and therefore we have here an abstract of the premises. 1. Heirs of God. The Lord himself is the portion of the saints' inheritance (Psa 16:5), a goodly heritage, Psa 16:6.

— Matthew Henry's Concise Commentary (public domain)

And if children, then heirs--"heirs also." heirs of God--of our Father's kingdom. 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 16:24-25; Ti2 2:12).

— Jamieson-Fausset-Brown Commentary (public domain)