For now, we will put aside answering the question, when did Jesus become God, and state what is obvious from the scriptures. At the end of his forty days and forty nights fast, Jesus had changed. He was being prepared by God to assume the grave responsibility of saving humankind from eternal damnation.
As part of his preparation, we read in Luke 4:1-13 that Jesus was tempted by the devil. But, note that God allowed the devil to tempt Jesus as we read in Matthew 4:1 "Then Jesus was led by the Spirit into the desert to be tempted by the devil."
Aware that Jesus was hungry at the end of his fasting, the devil first tempted Jesus with food. He challenged Jesus to change stone to bread, as if to say, if you are God satisfy your hunger here and now: turn these stones into bread. To which Jesus said in Luke 4:4 "It is written, a man does not live on bread alone."
Failing to trick Jesus into turning stone to bread, the devil told Jesus that all the wealth of the world would be his if he would worship him. To which Jesus said in Luke 4:8 "It is written, worship the Lord your God and serve Him alone."
Now frustrated, the devil took Jesus to the highest point of the temple and commanded him to jump, quoting a passage from the scriptures to stoke Jesus' pride into jumping, but he told the devil in Luke 4:12 "Do not put the Lord your God to the test."
When Jesus did not fall prey to the devil's temptations, even though Jesus was hungry after the long fasting, or to the offer to become rich and comfortable, showed that Jesus had completely subordinated the cravings of his body to the will of the Spirit, and this qualified him to be the savior of humankind.
Still, we should not be deluded into thinking nothing more is needed on our part to guarantee our salvation. On the contrary, a proper reading of the scriptures would reveal that although, "Jesus the Man" died on the cross to save us from the penalty of our past sins, protection from the power of sin depends on living by the Word.
Which was why "Jesus the Man" died, so that we could receive "Jesus the Word" (John 16:5-7), who had already began teaching us in Matthew 22:37 that the most important spiritual principle in our salvation plan is to "Love the Lord your God with all your heart, and with all your soul, and with all your mind" and the next is to "Love your neighbors as you love yourself".
You may have noticed by now that significant as the death of "Jesus the Man" was, it was a means to an end. Because, regardless of how much sorrow we feel, how many tears we shed, or how much love we have for "Jesus the Man", as we envision him suffering for our sins; our salvation depends solely on loving "Jesus the Word", who is embodied in the gospel.