What does 'master' signify?
It shows beyond doubt that Jesus was a man - just as the rest of men since Adam as myriad NT texts show without equivocation. The devil knew exactly who Jesus was - the son of God (Matt 4), he also knew who God was and he wasn't confused as to who was who. The demons knew too.
Acts 19:15 And the evil spirit answered and said to them, “I recognize Jesus, and I know about Paul, but who are you?”
Luke 10:17 The seventy returned with joy, saying, “Lord, even the demons are subject to us in Your name.”
James 2:19 You believe that God is one. You do well; the demons also believe, and shudder
Matt 8:29 And they cried out, saying, “What business do we have with each other, Son of God? Have You come here to torment us before the time?”
The devil knew Jesus was the son of God and tempted him through pride to get leverage into his privileged status.
As Jesus was to be the second and last Adam (human), and die for all creation as per the plan set in motion before the world began, then he had to win a victory over the devil on just terms.
Jesus had to defeat evil as a man under the guidance and power God gave him through the spirit.
Satan knew he couldn't beat God at any challenge - but he might just have a slim chance with God's human son! This was his last opportunity.
If Jesus was somehow God - immortal, unable to be tempted, the whole challenge would be declared a foul and God would be justly accused of cheating.
So, Jesus, while he was 'in the flesh' could die and was fully subject to the 'law of sin and death'. Meaning... if he sinned, he could not be raised again.
That we are told, 'he was tempted in all things as we are, yet without sin', this would be totally pointless if Jesus could not sin. God cannot be tempted (James 1:13) but Jesus could be tempted.
For we do not have a high priest not being able to sympathize with our weaknesses, but one having been tempted in all things by the same way, without sin. Heb 4:15
Therefore, He had to be made like His brethren in all things Heb 2:17
At no time or place could God (on the construct that Jesus is God) ever find Himself a slave to death or being 'mastered' by death. The idea is preposterous!
Being tempted means being able to sin
He could have sinned, otherwise the whole temptation thing was a sham.
Let's be perfectly clear! If Jesus was tempted by the devil from at least the 40 days and nights (Luke 4:2); then EVERY day until his death was the potential to sin. i.e. to put his will before the Father's. This means that he was under the power of, and 'mastered' by the law of sin and death that whole time.
Not just while he was in the grave! (as some have surmised) While he was in the grave, he had already accomplished the victory! He could hardly be subject to death then and didn't have to wait till he was alive again - 'It was already finished!'
That is why he prayed to the Father with 'loud crying' to be saved from death - NOT death on the cross, but the eternal death he would be subject to should he have slipped up and put his will before God's just once!
In the days of His flesh, He offered up both prayers and supplications with loud crying and tears to the One able to save Him from death, and He was heard because of His piety. Heb 5:7
Jesus was legally able to live again the moment he died - 'It is finished!' he cried.
When was death defeated?
so that by His death He might destroy him who holds the power of death Heb 2:14
Sin and Death was defeated the moment he died - not when he was raised. (Having maintained his sinlessness to the final moment) Being in the grave was a formal matter of the 3 days and nights - to fulfil the sign as prophesied. He said, he had authority to put his life down and take it up again - only because he had accomplished his mission perfectly and death could not hold him. (Jesus did not raise himself - as we are told quite consistently in over 30 places)
He was now the master of death!
Jesus, the man appointed by God, who made Jesus both Lord and Christ (Acts 2:36) born of Mary and lived to die - all according to God's brilliant plan in order to free all from the slavery of death, which he also submitted to for our sake.
Heb 2:14 Now since the children have flesh and blood, He too shared in their humanity, so that by His death He might destroy him who holds the power of death, that is, the devil.
Jesus 'rendered powerless him who had the power of death' If he did this, it was clearly not so before hand. The devil had a power of sin and death, which Jesus was under at every moment until his death.
God is never subject to the devil - Jesus, while in the flesh, was!
It's been suggested that Jesus voluntarily became obedient to death itself.
having been found as a man, he humbled himself, having become obedient unto death -- death even of a cross Phil 2:8 Youngs
He was obedient to his Father - not death! Jesus was subject to 'the law of sin and death', while he was able to sin... then death could hold him permanently. Jesus was obedient to the Father in all matters - up to and including, death on the cross.
Remember, Jesus had his own will - which differed from the Father's will.
John 6:38 For I came down from heaven, not to do mine own will, but the will of him that sent me.
Luke 22:42 yet not my will, but yours be done
Even though Jesus was God’s Son, he learned obedience from the things he suffered. Heb 5:8