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We have assurance of salvation through our putting our trust (aka faith) in what God himself accomplished on our behalf, rather than our putting our trust in our being 'good enough' in-and-of ourselves to merit eternal life. The righteousness that we need for salvation is a righteousness that we cannot earn as if it were a salary, nor can we purchase it as if it were an asset. It is a gift, totally unearned and undeserved by any of us, but given to us freely through the compassion which God has shown for us:
Now we know that whatever the law says, it says to those who are under the law, so that every mouth may be silenced and the whole world held accountable to God. Therefore no one will be declared righteous in his sight by observing the law; rather, through the law we become conscious of sin.
But now a righteousness from God, apart from law, has been made known, to which the Law and the Prophets testify. This righteousness from God comes through faith in Jesus Christ to all who believe. There is no difference, 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. God presented him as a sacrifice of atonement, through faith in his blood. He did this to demonstrate his justice, because in his forbearance he had left the sins committed beforehand unpunished - he did it to demonstrate his justice at the present time, so as to be just and the one who justifies those who have faith in Jesus. (Romans 3:19-26,NIV)
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What then shall we say? That the Gentiles, who did not pursue righteousness, have obtained it, a righteousness that is by faith; but Israel, who pursued a law of righteousness, has not attained it. Why not? Because they pursued it not by faith, but as if it were by works. They stumbled over the "stumbling stone." As it is written:
"See, I lay in Zion a stone that causes men to stumble and a rock that makes them fall, and the one who trusts in him will never be put to shame." (Romans 9:30-33,NIV)
And why did God see to it that we can obtain the righteousness we need for eternal life by earning it for us himself, rather than requiring that we earn it through our obeying a set of laws? It's due to a flaw in our psyche that makes it impossible for us to obey a set of laws:
We know that the law is spiritual; but I am unspiritual, sold as a slave to sin. I do not understand what I do. For what I want to do I do not do, but what I hate I do. And if I do what I do not want to do, I agree that the law is good. As it is, it is no longer I myself who do it, but it is sin living in me. I know that nothing good lives in me, that is, in my sinful nature. For I have the desire to do what is good, but I cannot carry it out. For what I do is not the good I want to do; no, the evil I do not want to do - this I keep on doing. Now if I do what I do not want to do, it is no longer I myself who do it, but it is sin living in me that does it.
So I find this law at work: When I want to do good, evil is right there with me. For in my inner being I delight in God's law; but I see another law at work in the members of my body, waging war against the law of my mind and making me a prisoner of the law of sin at work within my members. What a wretched man I am! Who will rescue me from this body of death? Thanks be to God - through Jesus Christ our Lord! (Romans 7:14-25a,NIV)
What St. Paul described in this passage is now called 'paradoxical intention.' There's even a saying which describes it: "The harder you try, the faster you fail." Psychologists have to take it into account in the treatment of their patients, or else the treatment is doomed to failure. It 'boils down' to our only succeeding in doing whatever comes naturally to us. If we try to do something, such as follow a set of laws as they should be followed, we will inevitably fail.
God has taken this flaw into account. So instead of insisting that we obey a set of laws in order to show our gratitude to him for his Son's sacrifice of atonement, he again has taken the initiative by implanting within us a new nature. And it is through this new nature that we live our lives 'in synch' with what he wants of us, rather than its being through our keeping any set of laws perfectly (which we could never accomplish):
So I say, live by the Spirit, and you will not gratify the desires of the sinful nature. For the sinful nature desires what is contrary to the Spirit, and the Spirit what is contrary to the sinful nature. They are in conflict with each other, so that you do not do what you want. But if you are led by the Spirit, you are not under law.
The acts of the sinful nature are obvious: sexual immorality, impurity and debauchery; idolatry and witchcraft; hatred, discord, jealousy, fits of rage, selfish ambition, dissensions, factions and envy; drunkenness, orgies, and the like. I warn you, as I did before, that those who live like this will not inherit the kingdom of God.
But the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, gentleness and self-control. Against such things there is no law. Those who belong to Christ Jesus have crucified the sinful nature with its passions and desires. Since we live by the Spirit, let us keep in step with the Spirit. Let us not become conceited, provoking and envying each other. (Galatians 5:16-26,NIV)
The motivations which are 'the acts of the sinful nature' are those which dictate our actions before we accept God's gift of salvation. These motivations lie at the core of all sinful acts, and no act which has any of them as its impetus can be identified as other than sinful. This includes even those acts which might appear to be righteous, or even pious.
In order for our actions to be 'in synch' with God's will, their origin must also be 'in synch' with his will. And in order for that to take place the origin must be the motivations which are 'the fruit of the Spirit'. Just as we can identify the motivations which are 'the acts of the sinful nature' as being the impetus for all sinful acts, we can also identify the motivations which are 'the fruit of the Spirit' as being the impetus for all righteous acts. In fact, the spirit of the law is contained in these 9 'fruit'. That's why the words which immediately follow their listing are, "Against such things there is no law."
But just as our salvation is to be seen as a gift, our new nature (the fruit of the Spirit) is also to be seen as a gift. As a result of our genuinely accepting the salvation which God offers freely to us, we also receive his Spirit, who brings with him this new nature. So for both our salvation and our proper Christian behavior after accepting that salvation the credit must be seen as God's alone.
