God, My FatherSýnishorn

All those led by the Holy Spirit are children of God. By accepting the fatherhood of God, we also receive the Holy Spirit as the perfect guide for our lives. You should not let yourself be carried away by carnal appetites, nor even by what you perceive with the five senses. You should be led by what the Holy Spirit has to say to you.

Verse 15 reminds us that we are not slaves but that we have been adopted.  We  know that we are children because the Holy Spirit reveals it to us, and bears witness that we are CHILDREN OF GOD.