Saying that we are “children of God” and “heirs” at the same time is a wonderful thing. Imagine for a moment that the president of your country decides to be your best friend, the first thing you would think is why you, that you do not deserve it, etc. So this is much bigger, because it’s not about a president or famous person, it’s about the most imaginable being that exists.
We are children and heirs of God, and so the Bible describes it:
15 For ye have not received the spirit of bondage again to fear; but ye have received the Spirit of adoption, whereby we cry, Abba, Father.
16 The Spirit itself beareth witness with our spirit, that we are the children of God:
17 And if children, then heirs; heirs of God, and joint-heirs with Christ; if so be that we suffer with him, that we may be also glorified together.
Have we received a spirit of slavery? No, but Jesus Himself has given us a spirit of freedom by believing fully in Him. So, we are highly confident people in the storm, as well as when the disciples were in the boat in the roughness of the sea, but Jesus was with them, why fear if Jesus was there?
Verse 16 says that the Spirit of God testifies to our spirit that we are children of God. It is not going to church that testifies that we are children of God, it is not offering, missions, etc., although everything mentioned is important. But this of being children of God is the same Spirit of God that testifies that we really are children of God.
And if we are His children, then we have an inheritance from our Father. And it is not an earthly inheritance, but an eternal dwelling place that He prepared for us.
I encourage you to continue trusting God fully, for we are His children, and if we are His children, we no longer fear, and we have an inheritance in Him.