"We know we remain in Him and He in us" (1 Jn 4:13). "When anyone acknowledges that Jesus is the Son of God, God dwells in him and he in God" (1 Jn 4:15). "God is Love, and he who abides in love abides in God, and God in him" (1 Jn 4:16). Four times in five verses God tells us that He — the Father, the Son, and the Holy Spirit — lives inside us who are living for Him.
The ever-present reality of the Trinity's dwelling in us should radically impact the way we live our lives. Realizing that God lives in us, we are transformed in the way we relate to others, see ourselves, make decisions, live our vocations, spend our time, pray, work, etc. For example, a pregnant mother's life is dramatically affected by having her child within her. It should affect us much more to have God within us not only for nine months but permanently.
However, "perhaps you yourselves do not realize that Christ Jesus is in you" (2 Cor 13:5). As when Jesus walked on the Sea of Galilee, the Trinity sometimes does not want us to know Their presence through our senses (Mk 6:48). We need the Holy Spirit to make us aware of the sometimes subtle, even hidden indwelling of the Trinity (1 Jn 4:13; 3:24). Only the Holy Spirit "knows what lies at the depths of God" (1 Cor 2:11) and what lies at the depths of our own being. Only the Holy Spirit can guide us to the truth (Jn 16:13) of the Trinity's dwelling in us. May the Holy Spirit, the only true Christmas Spirit, reveal to us that God is both with us and within us.