Matt. 4:4, ‘It is written, “Man shall not live by bread alone, but by every word that comes from the mouth of God”
Satan asks Jesus to turn stones into bread in order to prove that He is indeed the Son of God. Except, recall that the Father has just proclaimed Jesus to be His Son in Mt.3:17. Thus, Satan’s questioning of Jesus’ identity is a questioning of God’s word, and the challenge to turn stones to bread becomes a way not merely for Jesus to provide for Himself, but to prove in His own strength what God has already proclaimed by His word. For Jesus to live by every word that comes from the mouth of God means that He holds to and affirms the reality of what the Father has spoken about His own identity, the word that declared Him to be God’s Son, THAT is the word upon which Jesus lives. If He were to make stones into bread to prove His identity, He would be living not on God’s word, but—quite literally—on bread….the bread would be the final ‘proof’ of His identity, doing what God’s word alone should do.
We too live on the word that God speaks concerning us rather than the ‘bread’ of what we can achieve. Consider, God declares us to be His children in Christ (Jn 1:12-13), without condemnation (Rom 8:1ff), righteous in the Son’s righteousness (2 Cor 5:21), crucified with, raised with, hidden in, identified by Christ (Col 3:1-3), etc. Our identity as those united to Christ is something we receive from the mouth of YHWH, it is the word upon which we live, indeed, which is our very life. This life does not—cannot—come from the ‘bread’ of our own efforts, it comes only and wholly from the ‘word’ of YHWH’s mouth.
Just as the Son Himself receive His identity only as a word from the Father’s mouth and not as any work of His own, so too those in Christ receive their identities only as graciously spoken word, never as bread which they have made or which they can control. We are justified by word, not bread.
And the identity we receive from the mouth of the Father is the identity of the slain and risen Jesus, since we have been united to Him: This is my Beloved Son (or Daughter) in whom I am well pleased.