4 But He replied, It has been written, Man shall not live and be upheld and sustained by bread alone, but by every word that comes forth from the mouth of God.
Matthew 4:4 AMPC
The Just shall live by faith
Jesus has called us to live by the Word of God. Living by the Word means walking in faith, which is putting our complete trust and obedience in the Lord. We should not solely rely on physical food to sustain us or give us life. We rely on the Father who sustains us by His Word everyday. Looking a little deeper into what Jesus said to the devil, He said that command while He was fasting. He fasted for 40 days and did not eat anything. Jesus was letting the devil know that He was being sustained spiritually by the Word of His Father. We need to have the same trust in the Father and be led by His Holy Spirit. (Romans 8:14)
17 For in the Gospel a righteousness which God ascribes is revealed, both springing from faith and leading to faith [disclosed through the way of faith that arouses to more faith]. As it is written, The man who through faith is just and upright shall live and shall live by faith.
Romans 1:17 AMPC