That's why it is only God who saves.
"And you hath he quickened, who were dead in trespass and sins...For by grace are ye saved through faith and that not of yourselves: it is a gift of God: not of works, lest any man should boast. For we are his workmanship, created in Jesus Christ unto good works, which God hath before ordained that we should walk in them." -- Ephesians 2:1,8-10
It doesn't get clearer than that.
We are ALL fallen and dead in sin. We cannot save ourselves.
God, for His own good purpose, has chosen some from before the foundation of the world to redeem by paying the penalty for their sins with the sacrifice of Christ.
How does any of us know we are among those fortunate sinners whom God shelters "under the shadow of His wings?"
Here's how -- we read the Bible and understand. We hear God's instructions in Scripture and know the Holy Ghost is abiding with us.
We recognize we are fallen, dead in sin and trespass, unworthy of even a glimpse of God's splendor. We see that the color of our skin, our nationalities, our station in life, our personalities, our age, sex, political preference, choice of breakfast cereal, all mean nothing to our salvation. God chooses from all men and nations by His will.
Aware of ourselves as sinners, we fall to our knees in remorse, knowing we're completely responsible for every transgression we've committed. And we promise to sin no more, knowing we will fail in that promise, but also secure that God is with us always and will not let us fail Him.
We believe that Jesus Christ came to earth as God's gracious gift to us as payment for our sins. We read Christ's words and know that all truth exists withing His teaching. We believe Him when he says that the Son is the only door through which we can pass into the Father's house.
We recognize the unmerited gift that is our faith; that only God could open our eyes and ears to the truth of the Word. We love Him because He loved us first.
Our hearts are thus transformed by God, and we are born again.
We then are commanded to go out into the world, preaching God's glory, because God uses each of us to His purpose. As RnMom has said over and over -- only God saves. But God expects the Word to be spoken clearly and accurately, so that He might use its message to touch whatever other hearts He chooses.
And finally we are content and secure in the knowledge that God is in control of His creation; and that nothing can loosen His hold of us.
Dogs cannot become cats. Cats cannot become dogs.
But sinners become the inhabitants of Heaven by God's eternal decree as part of His divine plan for creation, known by Him from before the foundation of the world.
Question: What is the chief purpose of man?
Answer: The chief purpose of man is to glorify God and enjoy Him forever. -- Westminster Shorter Catechism