Let's open our Bibles to Romans 8:14-16.

If the Spirit is leading us to kill our sin and confirming to us the promises of the gospel, then we can be assured that we are children of God

If the Spirit is leading us to kill our sin, then we can be assured that we are children of God (v. 14)
No one who is living according to the flesh kills his sin on the heart level
John Murray: The activity of the believer is the evidence of the Spirit's activity and the activity of the Spirit is the cause of the believer's activity
We have been born into God's family through the Spirit who imparts new life to us
Charles Hodge on adoption:
There is similarity of disposition, character, or nature
"Sons" are the objects of special affection
"Sons" have a title to some peculiar dignity or advantage
If the Spirit is confirming the promises of the gospel to us, then we can be assured that we are children of God (vs. 15-16)
The Spirit confirms the Gospel promise that through adoption we become children of God (v. 15)
Adoption is relatively rare, occurring only three other times with reference to Christians: Rom. 8:23; Gal. 4:5; and Eph. 1:5
William Barclay on adoption in Roman Society:
The adopted person lost all rights in his old family and gained all the rights of a legitimate son in his new family
It followed that he became heir to his new father's estate
In law, the old life of the adopted person was completely wiped out; for instance, all debts were canceled
In the eyes of the law he was absolutely the son of his new father
The result of our adoption is that by the Spirit we cry out, "Abba! Father!"
Jesus taught us to pray, "Our Father..."
J. I. Packer: "Father" is the Christian name for God
The Spirit confirms the Gospel promises to us through His inner witness
No one seeks God; only the Spirit of God bears witness to your spirit that you are a child of God