Rom 3:20, Rom 3:27 & 28, Rom 4:3, Rom 4:5, Rom 4:6, Rom 9:32, Rom 11:6, Gal 2:16, Gal 3:6, Gal 3:11, 2 Tim 1:9, Titus 3:5.
Each of the following verses testify that justification is by faith:
John 3:16, Rom 3:22, Rom 3:24, Rom 3:26, Rom 3:28-30, Rom 4:3, Rom 4:5, Rom 4:11, Rom 4:16, Rom 5:1, Rom 5:9, Rom 9:30, Rom 9:33, Rom 10:4, Rom 10:9-10, Gal 2:16, Gal 2:21, Gal 3:5-6, Gal 3:8, Gal 3:14, Gal 3:22, Gal 3:24, Eph 1:13, Eph 2:8, Phil 3:9, 1 Tim 1:16
…and the following verses tell us that salvation is a gift (one does not earn a 'gift'):
Rom 6:23, Rom 1:8, Rom 4:5, Rom 4:16, 1 Cor 2:2, 1 Cor 15:6, 1 Cor 15:14, Eph 1:15, Phil 2:17, Col 1:4, Col 2:5, 1 Thess 1:8
Justification, which is the crediting of God's righteousness to the believer, is what produces the result of eternal life for the believer that Christ promises. If salvation could be lost by the failure to meet some standard of obedience or morality, then the justification that a person received upon placing their trust in Christ would have to be maintained by continually meeting that standard, whatever it may be. But this would mean that the fourteen passages above would have to be completely wrong.
Furthermore, note that Gal 3:10 states:
For as many as are of the works of the Law are under a curse; for it is written, cursed is everyone who does not abide by all things written in the book of the law, to perform them.
So, if we want to rely on a system of works to provide our justification, we've already lost. There are 613 laws in the Mosaic Law, and according to Gal 3:10, if you fail to obey just one of those laws in your lifetime, you are cursed. It's too late… with respect to obeying the Law, we're all done for.
And anyway, in Romans 6:14 Paul makes it very clear that we have been released from the law:
For sin shall not have dominion over you: for ye are not under the law, but under grace.
In addition to all of this, in Romans 11:6 Paul tells us that grace and works are mutually exclusive. They cannot be blended in any way. Thus, it cannot be said that justification is by grace plus a little works here or there.
But if it is by grace, it is no longer on the basis of works, otherwise grace is no longer grace.
So if we add just one "work" to grace, we've removed grace from the equation completely. There's no combining the two systems; they are incompatible.
But once again, this is not to be understood as any sort of command against works. The only point here is to emphasize once again that works is not part of the salvation equation. Once a person is saved, they are expected to behave themselves and do good things, etc. But, not all do… at least not right away, and whether they do or don't does not have any impact on whether they are saved. Remember Romans 4:5:
But to him who does not work but believes on Him who justifies the ungodly, his faith is accounted for righteousness,
This verse demonstrates that even the person who does not work is justified and granted God's perfect righteousness. It also demonstrates that believing, or putting your trust in something, does not fall under the category of work. Works cannot be part of the salvation equation in any way, shape, matter or form. Salvation is a gift and it does not depend upon you.