Justification by faith alone

For all have sinned and fall short of the glory of God, and are justified freely by his grace through the redemption that came by Christ Jesus.
One of the most famous song in the world is Amazing Grace.
Amazing grace, how sweet the sound that saved a wretch like me.
I once was lost, but now am found; was blind, but now I see
Twas grace that taught my heart to fear and grace my fears relieved.
How precious did that grace appear the hour I first believed
Every person has sinned, and broken God's law. Each person has turned away from God. The outcome of our sin is death - eternal separation from God.
Because we have turned away from God, there is nothing we can do to bring us back to him. No amount of good works will ever be enough to undo the sin we have done.
On the Cross, Jesus took our sin upon himself, and died for us. This was the only way that our sin could ever be taken away from us.
We can be made righteous in God's eyes (that is, justified) by our faith - when we accept Jesus Christ as our Lord and believe that he died for us. This belief wipes away all our sins - past, present and future. We are no longer eternally separated from God, but are now his children!
This faith is all that is needed for our salvation. There is nothing we can or need to do to earn our salvation - faith alone is sufficient.
Grace is this free, unmerited gift of salvation from God.
Good works are not required for our salvation. However, faith in Christ will lead to good works.
John 3:16 is probably the most well known Bible verse in the world.
For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life
Luke wrote in Acts 13:38-39

Paul said, "Therefore, my brothers, I want you to know that through Jesus the forgiveness of sins is proclaimed to you. Through him everyone who believes is justified from everything that could not be justified from the law of Moses."
Paul wrote in Romans 3:28

For we maintain that a man is justified by faith apart from observing the law.
Again Paul wrote in Galatians 2:16

We know that a man is not justified by observing the law, but by faith in Jesus Christ. So we, too, have put our faith in Christ Jesus that we may be justified by faith in Christ and not by observing the law, because by observing the law no-one will be justified.
Throughout the New Testament the theme is clear. There is nothing we can do to gain everlasting life with God. There is no way to ever earn this because we have sinned and turned from God. But because of Christ there is nothing we need to do. We can be cleaned from our sin (justified) and can have everlasting life with God freely, as a gift. This is grace.

The first words of the well known book called, "What's so amazing about grace" are

There is nothing we can do to make God love us more
There is nothing we can do to make God love us less.
I think that's pretty amazing too.