There were rewards for the Hebrews' keeping the laws and commandments of Torah, which includes the Ten Commandments. But the rewards were to be given them in this life:
If you pay attention to these laws and are careful to follow them, then the Lord your God will keep his covenant of love with you, as he swore to your forefathers. He will love you and bless you and increase your numbers. He will bless the fruit of your womb, the crops of your land - your grain, new wine and oil - the calves of your herds and the lambs of your flocks in the land that he swore to yuor forefathers to give you. You will be blessed more than any other people; none of your men or women will be childless, nor any of your livestock without young. The Lord will keep you free from every disease. He will not inflict on you the horrible diseases you knew in Egypt, but he will inflict them on all who hate you. (Deuteronomy 7:12-15,NIV)
That's it; there is the contract which God entered into with the Hebrews. They obeyed Mosaic Law in this life (including the Ten Commandments). They were rewarded in this life. It was a very pragmatic, here-and-now agreement.
By the time of Jesus' sojourn among us there were those who (1) believed that there was an afterlife, and (2) believed that we could 'buy our own ticket to heaven' if we obeyed the Mosaic Law scrupulously. Both Jesus Christ and St. Paul 'shot down' that belief:
"Suppose one of you had a servant plowing or looking after the sheep. Would he say to the servant when he comes in from the field, 'Come along now and sit down to eat'? Would he not rather say, 'Prepare my supper, get yourself ready and wait on me while I eat and drink; after that you may eat and drink'? Would he thank the servant because he did what he was told to do? So you also, when you have done everything you were told to do, should say, 'We are unworthy servants; we have only done our duty.' " (The Gospel of St. Luke 17:7-10,NIV)
What we do on behalf of God's kingdom is to be seen by us as nothing more than what we are supposed to do. There is to be no reward expected except the successful completion of whatever tasks are set before us.
But we can't even do everything that God would have us do; God himself knows that much better than any of us do. So how do imperfect people come before a God that demands absolute perfection and survive? We survive because God himself has provided for us the very righteousness which he requires of 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 justfies those who have faith in Jesus. (Romans 3:19-26, NIV)
The only righteousness which is acceptable to God is the righteousness which comes from God. This cannot be augmented, nor can it be set aside. If we want salvation we must accept it as he offers it.
And what are we to do in exchange for this righteousness? We are to accept yet another gift from God, namely, the Holy Spirit. It is only through our permitting him to create in us a new heart that we can serve God. And the Holy Spirit does not come to us empty-handed:
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)
There is our 'Christians Code of Conduct'. But it is a code which literally requires the very real presence of the Holy Spirit in order to be a success. And how do we obtain the Holy Spirit? We obtain him by accepting the righteousness which God offers freely to all those who wish to accept it.
Take a good look at that passage. Notice in the second paragraph (the acts of the sinful nature) how many of those 'acts' aren't actions, but rather are the undrlying evil emotions which culminate in evil acts being performed.
Now notice the third paragraph (the fruit of the Spirit). There we find that every 'fruit' is the underlying emotion which culminates in righteou acts being performed.
This 'Code of Conduct' is not a list of Do's and Don't's like we read in the Old Testament; far from it. This 'Code' requires the direct help of God himself, just as our salvation requires his direct help. The Holy Spirit must perform a 'heart transplant'. He must create in us a new heart that not only does not do what is sinful, but also does not want to do what is sinful. A new heart that not only wants to perform righteous acts, but also has the correct emotions undrlying the desire to perform those acts.
So even after we accept God's salvation for us, what we do in return is not to our credit; it's to God's. For just as we can only attain the righteousness necessary to be acceptable to God by receiving the righteousness given to us by God himself, even so we can only perform righteous acts that he would have us do if we permit the Holy Spirit to install in us the underlying emotions that lead to those acts being performed. So we aren't performing the acts; God is.
