The Spirit himself bears witness with our spirit that we are children of God, and if children, then heirs—heirs of God and fellow heirs with Christ, provided we suffer with him in order that we may also be glorified with him. (Romans 8:16-17)
The Spirit tells us that we are God's sons, and that as sons we are heirs. Heirs of what? Eternal life, the inheritance God gives to all his children (John 1:12; Romans 1:4). A son of God possesses the down-payment of that inheritance already - God's presence in the person of the Spirit - and so he can assume the rest is on its way (Eph 1:14).
And to recognize the mark of sonship, we have only to look to the Son himself. "Provided we suffer with him," Paul says. Suffering is the path to glory. Suffering is the way of sonship.
It is for discipline that you have to endure. God is treating you as sons. For what son is there whom his father does not discipline? Hebrews 12:7
This does not mean we should go out looking for (or worse, inciting) persecution. I dont' think external political or social persecution is even necessarily on Paul's mind here. Based on the context, the suffering he's referring to has everything to do with the war that wages constantly between our risen spirits and our dead flesh (see Rom 7). It's more than a struggle; it's suffering of the worst kind. A divided self. A holy self yoked to a sinful self. Paul says, "Wretched man that I am! Who will deliver me from this body of death?" (Rom 7:25). That's the sound of a man suffering.
Eternal life is for heirs only - for sons - for those who bear the image of the Father. Not for slaves (to sin). The Spirit testifies that we are sons, but our track record shows differently. How does this add up?
That's where the suffering comes in. Paul doesn't say, "if you are children, then heirs, provided you glide through life effortlessly, showing no sign of any struggle with sin whatsoever." Quite the opposite. He says, "you are heirs, provided you suffer with Christ."
Christ's suffering began with his resistance to sin, and to his own flesh:
In the days of his flesh, Jesus offered up prayers and supplications, with loud cries and tears, to him who was able to save him from death, and he was heard because of his reverence. Although he was a son, he learned obedience through what he suffered. Hebrews 5:7-8
He learned obedience through suffering the constant battle of sin. Do you think that battle made him feel dirty? I do. I think living in flesh with all its desires and cravings and wandering appetites made him feel agony. The devil came to Jesus at a mature age - about 30 years old (Luke 3:23ff). Jesus had already suffered decades in the flesh, and had encountered numerous temptations - all the same ones you and I have faced. So when the devil came to him and challenged him to prove he was the Son of God, you can imagine why he might have been tempted to do so.
Did I really resist that temptation?
Is God mad at me for being tempted?
Am I still pure, even though that thing I denied myself looked appealing at the time?
If I were really God's child, I shouldn't even WANT sin!
God, am I really your Son??
And yet, even in his weakness, hunger, and temptation, Jesus found security in the love of his Father for him.
Yes, I'm still your Son.
I don't have to prove it with power. You love me in my weakness.
You are my strength.
The fact that he obeyed perfectly means his war was harder than ours - not easier. He suffered sin more, not less, because he was sinless. And so he understands your suffering.
That is why Paul says that if you are an heir, you will "suffer with him." The inverse is, he suffers with you. He feels your pain, and he groans with you. He remembers what it felt like to battle that sin down.
You might feel like although he understands the pain of temptation, he could never understand the torment of failure. But he did; he suffered the torment of all our failure. He knows what it feels like to be cut off from God, to be so filthy with sin that you want to die, to be damned. Do you think that when he died carrying our sin that he didn't feel the guilt of it? He did. He knows your guilt because he held it in his heart on the cross, for you.
He suffered for your sin. And now, as you fight that battle on the ground, he suffers with you, too.
The point is, your struggle with sin, your weakness, your flesh, doesn't demote you or set you back from your walk with God into eternal life. It's not a diversion; it's the path. The struggle is the path. When you suffer the temptation or even the failure of sin - when it makes you sick and mad and miserable - you show that the Spirit is with you and is not letting you happily go on in your sin.
Of course, we want to have moments of victory, and we strive for this - so does he, on our behalf. But the moments of defeat can serve to assure us of our status as sons as powerfully as our moments of victory.
Because in defeat, we cry out to God with loud cries and tears for him to save us from death, and he hears us because of our reverence. Even in our struggle with sin, we can act like sons of God who know that our Father is powerful to save. We trust him. And then we get back up and keep walking like sons, pursuing holiness and glory. Because we know that he will get us there.
And the Lord your God will circumcise your heart and the heart of your offspring, so that you will love the Lord your God with all your heart and with all your soul, that you may live. Deut 30:6
Claim it. Write this down somewhere, in your Bible, on your desk, wherever: My struggle with sin is the path of sonship. Next time you feel down on yourself, next time you are tempted with sin, or even next time you fall, read yourself that note. Remember what it means. The agony you feel is evidence that you are his son. But don't stay in that agony - because the agony is only a means to an end, and the end is LOVE. Seek him. Cry out and ask him to reassure you of his love for you - ask a trusted fellow disciple of Jesus to reassure you of his love. Trust him - trust his love. And then get back up and keep walking, knowing that next time it happens he'll still be there.