The Scriptures clearly teach that Salvation is a free gift. We can do nothing to earn it. We are lost, alienated from God, cursed and deserving of physical and eternal death. Adam the first man acted for himself and for his posterity. He was the Federal Head of the entirety of the human race. His sin, whether the actual sin or the resulting curse, passes on to all of us, all of his descendants. As CS Lewis put it, being descendants of Adam we can hold our heads high and yet also hang our heads in shame. We bear the image of God and yet we're cursed by sin.
Like it or not, we're united to Adam, we're in Adam. We need a 2nd Adam to come along and create a new race, a new people. Old Testament Israel represented
both the 1st and 2nd Adams…a sort of repeat performance of the first and an anticipation of the second. This point is often missed, certain groups emphasizing the one to the exclusion of the other. The 2nd Adam comes and passes the test, completes the tasks and yet to rescue those under the curse of the 1st Adam he must pay the penalty of sin and make a way of reconciliation to God. Christ the 2nd Adam does this and now we are 'in' Him…united to Him, 'under' Him.
There's nothing we can do to affect this, to earn this, or even to maintain it. It truly is a gift. God out of pure love for His treasonous creatures did this for us. All of mankind is shown mercy, but only those of us who are saved can grasp the wonders, the breathtaking wonders of Grace. Its foundations in the Person and Work of Christ are rightly called amazing, and its application via the Holy Spirit…well, even the Angels are in awe of it.