Paul is telling us three things. He begins by saying, the Holy Spirit testifies to our spirit. In other words, the Spirit is telling us that we are children of God. And if we are children of God, then we also are joint heirs with Christ. Indeed, we suffer with Him. In other words, if we stick to the course and not drop out we can be glorified together with Christ.
You can’t compare what is coming ahead with what you may be going through now. You might be suffering and think “why God?” but Paul is telling us, hang in there, because when it’s over, there will be glory, sweet glory!