For Paul πίστις begins with the general idea of “the acceptance of the proclamation of God’s salvation activity in Christ” but makes it more theologically central:
Though faith does, indeed, find its basis in proclamation of God’s salvation activity, Paul thinks through the consequences of this event in a much more radical fashion: If God has acted for salvation once and for all in Christ’s cross, then the human response can only consist in obedient acceptance, in trust in God’s χάρις, and in receiving this gift with and in a life lived from within the gift itself. Thus πίστις belongs together with χάρις (Rom 4:4f., 16) and for the same reason is antithetical to ἔργα νόμου (Rom 3:28; 9:32; Gal 2:16) and to the νόμος understood as the principle of performance (Rom 3:21f.; Gal 3:12; Phil 3:9).
Thus faith is the only means to receive the gracious salvation gift of righteousness and Paul understands it to be the central goal of the OT message, as evidenced by his central use of Gal 15:6 and Hab 2:4 in Galatians and elsewhere. Faith thus is the central guiding principle of the Christian life as the individual and the larger community respond to the revelation of God in Christ and his continued work in their midst by the spirit.
In summary, it is fair to say that the NT takes the foundation laid in the OT with אמן and related terms and focuses the concept, making the response of the individual to the person of Christ revealed by God in the historical event of his death and resurrection a central topos for the term. This is shown well in Paul’s use of the terms in Galatians, of which most without doubt refer to the individual’s attitude of trust toward God. Exceptions would be 1:23, in which πίστις refers to Christian faith as a religious system per se, and 2:7, in which Paul uses πιστεύω with the sense “entrust.” The uses of πίστις in Gal 3:23, 25 might seem to be used differently that this majority within Galatians, perhaps referring to Christ himself, but upon closer inspection these uses also find their foundation in the individual’s attitude of trust towards God. They are personified and expanded without losing the central semantic anchor: They refer to a whole complex of events within salvation history which lead to faith, specifically the coming of Christ—his death, resurrection, and call to faith embodied in the Gospel.
“πίστις, πιστεύω,” EDNT 3:95. ↩