Christmas is all about the love of God for His people, for the world, and if we understand this, our response must be love for God and for His people and for the world.
John says that false prophets have gone out into the world, filled with the spirit of antichrist, who deny that Christ has come in the flesh (1 Jn. 4:1-3). But He who is in us is greater than he who is in the world (4:4). And we can tell who people belong to by the voices they listen to (4:5-6). John exhorts the Church to love one another because this is the mark of the life of God: God is love (4:7-8). Christmas and Easter are the great manifestations of the love of God (4:9-10), and when people get that, they love one another (4:11). This is because even though no one has ever seen God, God lives on earth in people who have been engulfed in the love of the Father and Son through the Spirit (4:12-14). We may not have seen God, but we have seen and testify that the Father sent the Son (4:12, 14-15). And this is why as we love Him and one another, God lives in us, so that “as He is, so are we in this world” (4:16-17). The opposite of love is fear, and fear is obsessed with punishment (4:18, cf. Mt. 25:46). We do not love Him out of fear but because He loves us (4:19), and our love of God is proven by our love for the people right in front of us (4:20-21). The proof of Christmas is in our love for God in Christ as He is present in those around us. This is why John is so worked up about the spirit of antichrist who denies Christ came in the flesh. In the incarnation we have seen God by the power of the Spirit, but John knows that this incarnating Spirit did not finish when Jesus ascended into heaven. Rather, that same incarnating, Christmas Spirit was poured out on all flesh at Pentecost. While Jesus is in heaven in His flesh, the Spirit is making Him present here in this world in and through the flesh of the saints.
Being a Christian means being a disciple; it means we have left everything for Jesus our Master. But our temptation in the Reformed tradition is to overemphasize the intellectual side of love, to the detriment of the rest of human experience. But the greatest commandment is to love God with all that we are, not just our minds (Dt. 6:5, Mt. 22:37), and Jesus insists that loving Him is central to following Him (Jn. 21:15-19). Paul connects knowledge, love, and the presence of God as well (Eph. 3:17-19). And notice all the plurals.
Paul picks up this theme in 2 Corinthians 5 as well where he is simultaneously defending his own credentials and assuring the Corinthians of the truth of the gospel. And the thing that connects these two is the Spirit of God who is their guarantee (cf. 1 Jn. 4:12-13). And this Spirit is the love of Christ compelling them and making them seem crazy (2 Cor. 5:13-14). This same Spirit pours out many different gifts, but it is the same obsessive love of the Trinity at work in all (1 Cor. 12-13). Therefore, Christmas means knowing the love of God in Christ and loving one another (1 Jn. 4:9-11).