There are two laws in existence, only two laws. You are either under the Mosaic law, or you are under the law of Christ. And most people, are clueless as to what each means because they refuse to follow a study that teaches what they mean and the differences between the two. They are two phrases that are easy to recite, but are generally not understood.
God is perfect, and God is the creator and owner of all that is. God is the glue that holds all of creation together. Without God, there would be no creation. There would be nothing.
To exist and live with God for all of eternity, one has to be perfect, as God is perfect.
Man is not perfect. We have all sinned, we have all failed, and therefore, in order to gain perfection, we have to do it ourselves, or have someone else do it for us.
The Mosaic law, and specifically the Ten Commandments, spell out the criteria for being perfect. But the Mosaic law, simultaneously spells out the criteria for doom.
No created being, is capable of keeping the Ten Commandments, therefore to try to live under the law, is a pursuit that will fail.
The law of Christ, is the law of grace.
Christ did all of the work to establish perfection for humanity, and Christ gets all of the credit. To live under the law of Christ, you simply believe in Jesus Christ. And that faith removes you from the law of Moses, which is nothing more than a law of self-effort, and places you under the law of Christ, which is His effort, not yours.
The grace provision, is that He did all of the work for salvation, and you did nothing except believe in Jesus Christ.
There is no merit to you, for believing, since there is no work involved in your believing. Jesus Christ did all of the work, and He deserves all of the credit.
So the contrast is in who does the work and who gets the credit.
To reject Jesus Christ, means that you believe that your own effort can accomplish salvation. And your own effort places you up against the Mosaic law, which clearly demonstrates that you are incapable of living a perfect life.
To believe in Jesus Christ, means that you look to Him for the requirements of salvation, and not to yourself.
Since you have believed in Christ, then sin is no longer relevant in your life with regard to salvation. And, being under grace, then sin has no further power over you, to prevent your salvation. Jesus Christ has already solved that, and eliminated sin as an issue.
Therefore, by believing in Christ, through faith, your eternal life is totally and completely in His hands, and no longer in the hands of anything or anyone else.
That is eternal security.
Once you have believed in Christ, you cannot turn back the clock and undo your belief. Once you have had a thought, you cannot undo that thought. It has already occurred.
Faith in Christ, is permanent, is irreversible, cannot be undone or changed. Faith is the ultimate guarantee, which was given to you by God, as a gift. And that is grace.