But what about the early Christians, specifically Paul and the other writers of the New Testament? Before looking at any of the other NT references to "faith" I just want to make the general comment that I feel that commentators often interpret the Gospels in the light of Paul and the other apostles, rather than interpreting Paul in the light of the Gospels. For example, it's common to define "Christianity", or "the Gospel", or to determine the fundamentals or first principles from the writings of Paul rather than the teachings of Jesus. This is such a common practice that Paul, rather than Jesus, has at times been described as the founder of Christianity (for example, in Gerd Ludemann's book Paul: the Founder of Christianity). By starting from a position that Paul's letters were written before the Gospels, C.S. Lewis concluded that the Gospel is not in the Gospels! Based on the claim that Paul's message was primarily about the cross Billy Graham wrote that "Jesus came to do three days work. Jesus came not primarily to preach the Gospel ... But He came rather that there might be a Gospel to preach." (For references see this article by Anthony Buzzard).
Some people might criticise these comments while themselves interpreting many of Jesus' sayings against a background of Paul's writings, rather than interpreting Paul as a follower of Jesus. So it appears that we sometimes have Paul in conflict with Jesus because we have approached his letters from the wrong end. I don't want to go off on a tangent (although I might write a digression about this later), but I'll just give one example of how this works in practice. The idea that women must be silent in church and have no leadership role is based solely on the writings of Paul while it seems that Jesus acted radically against custom and convention. Furthermore, it appears that Jesus, by example, taught the opposite of Paul: Jesus spoke with women in public (it was forbidden for rabbis to speak to a woman in public in case it appeared that he was teaching her!); a group of women followed Him, supported Him from their livelihood, and were part of His inner circle; and on one occasion Jesus said it was better for a woman to sit at His feet and learn from Him than to attend to domestic chores. Jesus raised the social status of women. From a casual reading of Paul's letters it appears that he contradicted himself: on one occasion he said there was no difference between men and women in Christ, while on another he said women should be silent in church. However, the apparent contradictions soon disappear when we interpret Paul in the light of Jesus' radical transformation of the place of women. While Jewish society and the religious leadership taught that women should not be educated in Torah, Paul, acting on Jesus' example, said "a woman should learn" (1 Tim 2:11) and should not exercise leadership until she has first had an education in spiritual matters. The idea that women should be in submission to men isn't in Paul's writings at all - these ideas came later from the likes of Tertullian and Augustine and Paul has been misquoted in order to defend them. Take the "church fathers" out of the picture and read Paul as a disciple of Jesus and we find him teaching something quite different. (My apology for going off on a tangent).
Coming back to faith we see that Paul built on the teachings on Jesus that "faith" means a practical trust in the power of God to provide, to meet physical needs and to save. Paul used the noun pistis 142 times, compared to 101 times in the rest of the NT. He used the verb pisteuo 54 times and the adjective pistos 33 times. Thanks to Paul's considerable teaching on the subject faith became established as of central importance in Christianity.
Tied in to his teaching about faith, Paul also emphasised that salvation is a gift of grace and cannot be earned. We cannot merit salvation - we can only trust in God to provide. When he writes that we are "justified freely by his grace through the redemption that came by Christ Jesus" and refers to Christ as "a sacrifice of atonement" he immediately adds that this is effected "through faith in his blood" (Rom 3:24-25). In other words, God has done everything necessary for our salvation - it is "freely by his grace" - there is nothing we can do to merit it. Our role is to trust (pistis) Him to save. The link between grace and faith is also made elsewhere (e.g. Eph 2:8 "For it is by grace you have been saved, through faith—and this not from yourselves, it is the gift of God"). As Leon Morris has written: "Grace is important in understanding faith, for it emphasises that salvation is a free gift, not a reward for human achievement of any sort, even as a reward for outstanding faith" (my emphasis - "Faith" in Dictionary of Paul and His Letters).
Here is where some people make a mistake. They rightly insist that salvation is not earned by good works, but they then insist with equal force that it is dependant on good doctrine!The main thrust of Paul's argument in Roman and Galatians is that both Jews and Gentiles are saved on the same basis - faith. He relies heavily on the example of Abraham and says that Abraham was accepted by God simply because he trusted God to perform what He had promised. Paul argued that Gentiles are accepted on the same basis: those who are "of faith" are the "sons of Abraham" and are "are blessed along with Abraham, the man of faith" (Gal 3:7-9).
Nowhere does Paul write of faith as something that can be acquired through learning. He describes it as a gift from God (1 Cor 12:9, Rom 12:3), as a fruit of the Spirit (Gal 5:22), and links it to the indwelling of Christ in the believer (Gal 2:20).
The expression "the faith" is often used by believers as way of referring to correct doctrine - the one, true, apostolic faith. It has become another way of speaking of a system of theology, or as a set of distinctive doctrines which separate one denomination from others. So the theological position of a church or denomination is often summarised as their defining "Statement of Faith". It has sometimes been claimed that conflicts, divisions, debates and arguments in defence of a doctrinal or theological position are actually a defence of "the faith" and are required by Jude 3 - "contend for the faith". Ironically, I have actually seen some ungodly behaviour defended on the basis that it was justified because it was in defence of "the faith".
However, Paul distinguished faith from knowledge (Eph 4:13) and emphasised that "my message and my preaching were not with wise and persuasive words, but with a demonstration of the Spirit's power, so that your faith might not rest on men's wisdom, but on God's power" (1 Cor 2:4-5). Nothing could be clearer: "faith" is not defended by persuasive arguments but through the demonstration of the power of God in our lives.
What did Paul mean then by "the faith" (with the definite article)? Leon Morris answers this well:
"So fundamental is faith that the term may be used to categorise the whole Christian way, and the expression 'the faith' comes into being, not simply as a way of referring to the trust in Christ that is so basic, but as a means of drawing attention to the whole body of teaching and practice that characterises the Christian group ... the whole Christian system of truth that so strongly emphasised the importance of faith ... the Christian way of belief that results from that trust" (ibid).Paul has the whole faith-based Christian life in mind when he writes "examine yourselves to see whether you are in the faith" (2 Cor 13:5), or "stand firm in the faith" (1 Cor 16:13). His emphasis was on the practical outworking of faith in Christian lives. So he wrote in Phil 1:27 "Whatever happens, conduct yourselves in a manner worthy of the gospel of Christ. Then ... I will know that you stand firm in one spirit, contending as one man for the faith of the gospel".
Paul was not appealing for doctrinal uniformity, but for steadfast and consistent Christian living.