21 We come to see the difference, then, which faith makes for us. Those who believe are transformed by the love to which they have opened their hearts in faith. By their openness to this offer of primordial love, their lives are enlarged and expanded. "It is no longer I who live, but Christ who lives in me" ( Gal 2:20). "May Christ dwell in your hearts through faith" ( Eph 3:17). The self-awareness of the believer now expands because of the presence of another; it now lives in this other and thus, in love, life takes on a whole new breadth. Here we see the Holy Spirit at work. The Christian can see with the eyes of Jesus and share in his mind, his filial disposition, because he or she shares in his love, which is the Spirit. In the love of Jesus, we receive in a certain way his vision. Without being conformed to him in love, without the presence of the Spirit, it is impossible to confess him as Lord (cf. 1 Cor 12:3).
Source: Lumen Fidei (Vatican.va)