Until Satan's power over mankind was destroyed, forgiveness would have been pointless, as man would have simply sinned again, almost immediately, and found themselves in the same decrepit, fallen state, still under the heel of Satan.
Gen 2:17 But of the tree of knowledge of good and evil, thou shalt not eat. For in what day soever thou shalt eat of it, thou shalt die the death.
Under God's most basic law, if you sin, you die.
The flip side of that most basic law is that a sinless man is not liable to die.
And no one (not even Satan) has the authority to take the life of a sinless man, let alone the only begotten son of the living God.
So Jesus had to become a sinless man, and he had to allow himself to be put to death, in order to destroy Satan's dominion over the earth and over mankind.
As Satan (the prince of this world) was ultimately responsible for using the Jews and the Romans to crucify the totally innocent and sinless Jesus,
Satan would have to answer to God for killing Christ.
Satan was subsequently judged by God, found guilty, and he was required to forfeit all that he had earlier gained from Adam's original sin.
The power and dominion over mankind and all the earth, that was originally given by God to Adam, then foolishly lost to Satan, was now awarded to the divine man, Jesus Christ, the new head of all mankind.
Here's what Jesus had to say about it:
Joh 12:31 Now is the judgment of the world: now shall the prince of this world be cast out.
Joh 12:32 And I, if I be lifted up from the earth, will draw all things to myself.
Joh 16:7 But I tell you the truth: it is expedient to you that I go. For if I go not, the Paraclete will not come to you: but if I go, I will send him to you.
Joh 16:8 And when he is come, he will convince the world of sin and of justice and of judgment.
Joh 16:9 Of sin: because they believed not in me.
Joh 16:10 And of justice: because I go to the Father: and you shall see me no longer.
Joh 16:11 And of judgment: because the prince of this world is already judged.
Mat 28:18 And Jesus coming, spoke to them, saying: All power is given to me in heaven and in earth.
After Christ's atonement, while Satan no longer had any "official" dominion over the earth or over mankind, people could still make a free will choice to follow him, and to reject God, so mankind was still not "home free".
Knowing this, Jesus followed up his redemptive sacrifice by founding his church, which is the ordinary means for all to reject Satan, and to profess their faith and allegiance to God (via baptism).
The church, empowered and headed by Christ, now provides the reservoir of grace and forgiveness that we all need, to recover from our ingrained tendency to sin.
So, Christ could not have redeemed us simply by living. He had to die, in order to redeem us. There was no other choice.
Mar 10:44 And whosoever will be first among you shall be the servant of all.
Mar 10:45 For the Son of man also is not come to be ministered unto: but to minister and to give his life a redemption for many.
Luk 9:22 Saying: The Son of man must suffer many things and be rejected by the ancients and chief priests and scribes and be killed and the third day rise again.
And only Jesus could have done it, because it was impossible for any other man to be born without the inherited stain of original sin.
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