Obedience.
Faith without the corresponding action [work] of obedience is dead, and can not justify.
Faith which comes by hearing God's word [voice; rhema] must find it's life in the act of obedience, which by any definition is being a doer of the word we hear.
Justified means; Made righteous; declared to be righteous. as well as shown to be righteous.
Believe carries the idea of committed trust, as well as obedience.
Unbelief is disobedience.
Abraham showed that he believed by obeying what he heard God tell him to do.
What then shall we say that Abraham our father has found according to the flesh?[a] 2 For if Abraham was justified by works, he has something to boast about, but not before God. 3 For what does the Scripture say?“Abraham believed God, and it was accounted to him for righteousness.”[b] 4 Now to him who works, the wages are not counted as grace but as debt. Romans 4:1-4
Some think this somehow means Abraham did nothing.
Abraham was first justified, when he obeyed the Lord to get out of his father's house, to a country that God would later show him.
By faith Abraham obeyed.
By faith Abraham believed, and showed that he believed by obeying what he heard.
Likewise, with the Gospel, we believe the Gospel, by obeying the Gospel.
Repent, for the kingdom of God is at hand.
If we don't obey with the corresponding act of repenting, then our faith that we received when we heard the Gospel is dead, and can not save.
It is by confessing with our mouth, Jesus as Lord, that faith finds it's expression in obedience and is able to save.
In other words, we can believe that Jesus died for our sins, and God raised Him from the dead, but if we don't turn from Satan as our lord [master; the one we obey] by confessing Jesus as Lord, then our believing all by itself, without the act of obedience is dead.
That's why Paul said it is both believing together with the act of obedience, that saves.
9 that if you confess with your mouth the Lord Jesus and believe in your heart that God has raised Him from the dead, you will be saved. 10 For with the heart one believes unto righteousness, and with the mouth confession is made unto salvation.
Romans 10:9-10
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