4 Now [de] to him that worketh [ergazomai] is [logizomai] the reward [misthos] not [ou] reckoned [logizomai] of [kata] grace [charis], but [alla] of [kata] debt [opheilema]. KJV-Interlinear
4 Now to
the one who works, his wages are not counted as a gift but as his due. ESV
Faith is contrasted with works, in that Paul makes it clear that is a person works, then his pay is not only expected but mandatory. If a person works then he has earned his pay.
One cannot contract for another to work, and then pay him only out of courtesy or as a favor. The work is done, and the pay is obligated.
And so if people were capable of working for their salvation, then God would be obligated to pay them with salvation.
And in so doing they would be able to make the argument that they were equal with God and as capable as God, with regard to making themselves pure and perfect.
But since the fall of Adam and Eve, man has repeatedly demonstrated that he is not capable of doing what is right throughout his life, but will always fail sooner or later.
Satan, formerly Lucifer, is the proof that even a higher creature, is not capable of righteousness.
He has greater capabilities, has had a far longer life, and yet he not only failed, but has repeatedly failed and even holds onto his failure in an effort to justify himself and destroy God.
So, man is incapable of purity, and therefore incapable of making himself right, and is therefore not, nor ever will be, equal with God. And, this inability to save himself, means that he needs someone else to perform the work of salvation, for him.
In this principle, no work means no pay, and when someone else does your work for you, then you are not entitled to their pay, but may only receive a benefit of that pay, through a gift given to you, by that other person.
And of course Jesus Christ is the one who worked for salvation in our behalf, and His only terms for our receiving the benefit of that gift is to believe in Him, in His work, in who He is, namely God and man, savior, and so forth. Believing in Christ incorporates all that He is and does and will do, for us.
Believing in Christ also is a recognition of the Fathers plan that we must approach Him through Christ. Which is to say that we accept the work of Christ, and have gained access to the Father by means of that work.
To presume that we have direct access to the Father, is what most religions do, by conveniently stepping around Jesus, ignoring Him, and presuming that we have right and privilege to the Father in spite of Christ.
To reject the Fathers plan, is to disrespect that plan, and therefore to reject the Father.
And works and religions, and philosophies do just that.