Law and Justice Satisfied

How do we see that David speaks by Christ's Holy Spirit as his own words? Is salvation an experience? Is it something you get or is it given? Why do some use the law in conditional, legalistic preaching to whip the people into conformity? How is death and condemnation synonymous? What is the only way a Holy God can justify sinners? Does personal obedience have anything to do with salvation? How does the law demand love? How was Satan a murderer? How did sin enter the world by Adam both in deciding to turn from God by not stopping Eve as her representative and because he carried the seed of all humanity and infected it?