Within Christianity, faith, in one sense, is often discussed in terms of believing God's promises, trusting in his faithfulness, and relying on God's character and faithfulness to act. Some denominations believe in the New Covenant and in the doctrine of salvation by faith alone (sola fide). According to most Christian traditions and denominations, Christian faith requires a belief in the resurrection of Jesus, and the Agony in the Garden which Jesus states is the plan of God the Father.
Since the Protestant Reformation of the 16th century the meaning of the term "faith" has been an object of major theological disagreement in Western Christianity. The differences have been largely overcome in the Joint Declaration on the Doctrine of Justification (1999). The precise understanding of the term "faith" differs among the various Christian traditions. Despite these differences, Christians generally agree that faith in Jesus lies at the core of the Christian tradition, and that such faith is required in order to be a Christian.
Some of the definitions of faith in the history of Christian theology have followed the biblical formulation in : "Now faith is the substance of things hoped for, the evidence of things not seen (NKJV)".
According to Berkhof, "Faith" is one step beyond religion, as it is the establishment of God's relationship to his people. Christian faith builds on Old Testament faith because the promises of the Old Testament are accepted as being fulfilled in the ministry, suffering, death, and resurrection of Jesus Christ.
New Testament
The word "faith", translated from the Greek πίστις (pi'stis), was primarily used in the New Testament with the Greek perfect tense and translates as a noun-verb hybrid; which is not adequately conveyed by the English noun. The verb form of pi'stis is pisteuo, which is often translated into English versions of the New Testament as 'believe'. The adjectival form, pistos, is almost always translated as 'faithful'. The New Testament writers, following the translators of the Septuagint (Greek Old Testament) rendered words in the Hebrew scriptures that concerned 'faithfulness' using pi'stis-group words. The pi'stis-group words are most appropriately translated into English by a range of words, depending on the context in which they occur. In both the New Testament and other Greek texts, pi'stis describes connections of firmness that can form between a wide variety of entities: people, traditions, practices, groups, purposes, facts or propositions. The appropriate English translation is often evident from the relationship between the two entities connected by pi'stis. The pi'stis-group words in the New Testament can thus be interpreted as relating to ideas of faithfulness, fidelity, loyalty, commitment, trust, belief, and proof. The most appropriate interpretation and translation of pi'stis-group words in the New Testament is a matter of recent controversy, particularly over the meaning of pi'stis when it is directed towards Jesus.
Specific verses
This passage concerning the function of faith in relation to the covenant of God is often used as a definition of faith. (hy-po'sta-sis), translated "assurance" here, commonly appears in ancient papyrus business documents, conveying the idea that a covenant is an exchange of assurances which guarantees the future transfer of possessions described in the contract. In view of this, James Hope Moulton and George Milligan suggest the rendering: "Faith is the title deed of things hoped for".
In recent decades, scholars have researched what pi'stis meant in the social context of the New Testament writers. Several scholars who have studied the usage of pi'stis in both early Greek manuscripts and the New Testament have concluded that 'faithfulness' is the most satisfactory English translation in many instances. This recent research has prompted some to argue that New Testament faith and belief in Jesus should be understood in terms of faithfulness, loyalty, and commitment to him and his teachings, rather than in terms of belief, trust and reliance.
Catholic Church
The Catholic Church considers that faith is twofold. Objectively, faith is the sum of truths revealed by God in Scripture and tradition and which the Catholic Church presents in a brief form in its creeds. Subjectively, faith stands for the habit or virtue by which these truths are assented to.