The justifying act is trusting assent to the whole Word of God
So the formal act of saving justifying faith — the condition by which we are proximately disposed to justification, and through which Scripture says we are justified — is best stated as practical fiducial assent to the whole Word of God, especially the Gospel promises.
That assent embraces every act that constitutes living saving faith; it is God's appointed means of sharing Christ's benefits and the condition to which Scripture attaches remission and the rest of salvation.
As soon as such assent is present, for Christ's sake one ceases to be under wrath and begins to be accepted to eternal life — which is justification itself. No one shares those benefits before that assent.
Latin
CXIII. Proinde meo judicio, nihil dici potest convenientius, quam actum formalem fidei salvificae & justificantis, qui proponitur tanquam salutis conditio, quo proxime disponimur ad justificationem, & per quem propterea Scriptura dicit nos justificari, nihil, inquam, convenientius dici potest, quam actum illum esse assensum practicum atque fiducialem adhibitum toti Dei Verbo, sed speciatim atque potissimum Evangelii promissionibus. Nam assensus ille practicus atque fiducialis sub se complectitur, & secum involvit omnes actus vivam & salvificam fidem constituentes: Et medium est a Deo destinatum per quod beneficiorum Christi participes simus; ac conditio illa cui Scriptura promittit remissionem peccatorum, & reliqua quibus constat hominis salus. Denique statim atque talis assensus in homine reperitur, propter Christum cessat esse irae divinae obnoxius, & incipit esse Deo gratus & acceptus ad vitam aeternam, quibus in rebus consistit hominis justificatio. Nec quisquam beneficiorum istorum particeps est, priusquam tali assensu praeditus sit.