What does this mean?
Does this mean that before jesus' resurrection, God never had victory over the power of death and sin?
To answer this question we have to go back to the beginning and a theological point over which Muslims and Christians disagree. That is--What is our natural state?
From the Christian point of view Adam and Eve were created perfect? They enjoyed complete and perfect fellowship with God as well. They did not know both good and evil, but were created knowing only good. But they were allowed freedom of choice. They could choose to serve God and conform their will to His or to assert their own will over God's will and usurp the divine will of God in their lives with their own will. This latter is what they chose. The act of choosing one's own will over God's will is itself a sinful act. And such sin lead immediately to the separation of Adam and Eve from the fellowship they were created to enjoy with God -- a type of death, spiritual death which would subsequently be followed by physical death as well.
As fallen creatures, marred at their spiritual core, and living in a world separated apart from continued fellowship with God as they had once had they now lived in a fallen world. It is into this fallen world, with a fallen nature similarly marred as their parent's nature was that all their children were born. (Again, I know Muslims disagree with this assessment, but you have asked for my Christian understanding of events.) And so, death was what all persons knew. God still had power, as one saw with healings and other miracles to step into this world and change the natural order even to bring people back to life, but our very natures had been changed so that death, not just physical death but also spiritual death, and permament separation from God was to be normative for us. Thus it is that Jews believed that they dead resided in Sheol, which is quite apart from God.
Yet there was for many also a belief in the ultimate promise of at least an eschatological resurrection when God would set all things right again. Many Jews of Jesus' day believed that this would be a result of the arrival of the Messiah who would usher in the Day of the Lord at the end times. Indeed, this is a belief that some (though not all it seems) Jews still hold and await. For Christians, we believe this day has come.
We see in the person of Jesus Christ, the triumph over death and the offering of a new way of connecting with God again. We believe that the old nature can be replaced with a new nature in which one is reconciled and made whole with God again through not our own work, but the work of Jesus Christ. And most specifically, his willingness to die our death that we might receive from him the offering of his life. So, in the words of all Muslims' favorite (sarcasm intended) Christian, St. Paul,
We were therefore buried with him through baptism into death in order that, just as Christ was raised from the dead through the glory of the Father, we too may live a new life. (Romans 6:4)
we know that the one who raised the Lord Jesus from the dead will also raise us with Jesus and present us with you to himself. (2 Corinthians 4:14)
So, from the time of the fall, till the time of the resurrection, the natural order was that for people who sinned (and even if you don't believe in original sin or inherited depravity, there is still the reality that we all do in fact commit sins) death reigned. But now, for those who are in Christ, as a result of his resurrection, life reigns. And this is why we say that it is by means of Christ's resurrection that we have victory over the power of sin and death.
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And Naidamar, I see one other thing of import in your question that I failed to address before. Whose victory are we talking about? As I said above, God has always had power over sin and death. They have no effect on him. Huamnity are the ones subject to them. So, what Christ accomplishes by means of his resurrection is to secure us victory where we previously had been defeated. God wins the victory for us which we could not win for ourselves.