We really are God's children

Romans 8:1-17
For what the law was powerless to do because it was weakened by the flesh, God did by sending his own Son in the likeness of sinful flesh to be a sin offering.  
And so he condemned sin in the flesh, in order that the righteous requirement of the law might be fully met in us, who do not live according to the flesh but according to the Spirit.
The mind governed by the flesh is death, but the mind governed by the Spirit is life and peace. The mind governed by the flesh is hostile to God; it does not submit to God’s law, nor can it do so.
But if Christ is in you, then even though your body is subject to death because of sin, the Spirit gives life because of righteousness.
For those who are led by the Spirit of God are the children of God.  The Spirit you received does not make you slaves, so that you live in fear again; rather, the Spirit you received brought about your adoption to sonship. And by him we cry, “Abba, Father.” 
God has done what the law could not do. We are no longer controlled by our human nature; we can live according to the Spirit.
The Spirit of God is alive and actively at work in everyone who belongs to Christ. Even our bodies, which still die, will be raised to life by God's Spirit.
It is his presence that convinces us we really are God's children.
Why?
The Holy Spirit has given us, whose mind governed by the Spirit. Even out body is subjected to death because of sin, the Spirit gives life.
We are slave  to God, but we do not live in fear, He is my heavenly father, we are his inheritance.
How?
God has done what the law could not do. We are no longer controlled by our human nature, we are live according to the Spirit. We really are God's children