This commentary from today's reading should serve as a reminder of what true communion with Christ is. Our communion with the Church leads us to communion with God. It is through an authentic communion that we come to experience true joy.
Feast: St. John, Apostle & Evangelist
From: 1 John 1:1-4
 That which was from the beginning, which we have heard, which we have seen with our eyes, which we have looked upon and touched with our hands, concerning the word of life-- the life was made manifest, and we saw it, and testify to it, and proclaim to you the eternal life which was with the Father and was made manifest to us-- that which we have seen and heard we proclaim also to you, so that you may have fellowship with us; and our fellowship is with the Father and with his Son Jesus Christ.  And we are writing this that our joy may be complete.
1-4. Since the time of the Fathers, these verses have been described as the prologue", like the prologue of the Fourth Gospel (Jn 1:1-18). In fact, there are many similarities in doctrine, style and even language between the two.
Both passages sing the praises of the mystery of the Incarnation: the Word of God who existed from all eternity, "from the beginning", became man (has been seen, heard, looked upon and touched) so that men might partake of divine life--might have "fellowship", communion, with the Father and the Son. Like the Gospel prologue, this one is written in a rhythmical way--"That which was..., which we have heard..., which we have seen...". And many of the ideas are the same--for example, the reference to "the beginning" (cf. Jn 1:1); the term "the Word" to refer to the second Person of the Blessed Trinity; the reference to "life" (cf. Jn 1:4).
As St Bede points out, "from the very start of the epistle we are being taught the divinity and, at the same time, the humanity of our God and Lord Jesus Christ" ("In I Epist. S. Ioannis, ad loc.").
1. "That which was from the beginning": although the pronoun used is neuter--as if to indicate the ineffable character of the mystery of Christ--the whole phrase refers not to a thing or an abstract teaching, but to the divine Person of the Son, who in the fullness of time was made manifest (v. 2), assuming a human nature. In other words, St John, as in his Gospel, is teaching that Jesus, a historical person (the Apostles have lived with him, have seen him, have heard him speak) is the eternal Word of God (cf. Jn 1:1 and note).
"That which we have heard,...seen...": all those references to perception by the senses show the Apostle's desire to make it clear that God really did become man. This may be because certain heretics were denying the Incarnation, or it may simply be that he thought it necessary to spell out this fundamental truth of our faith. He did so in the Gospel (cf., e.g., Jn 20:30-31); and in this letter we frequently find phrases like "Jesus Christ has come in the flesh" (4:2); "Jesus is the Christ" (2:22; cf. 5:1); "Jesus is the Son of God" (4:15; cf.5:1, 12,20).
We have recently been reminded that "the Church reverently preserved the mystery of the Son of God, who was made man, and in the course of the ages and of the centuries has propounded it for belief in a more explicit way"; moreover, what the Church teaches "concerning the one and the same Christ the Son of God, begotten before the ages in his divine nature and in time in his human nature, and also concerning the eternal persons of the Most Holy Trinity, belongs to the immutable truth of the Catholic faith" (SCDF, "Mysterium Filii Dei", 2 and 6).
2. St John introduces this verse by way of parenthesis to explain what he means by "the word of life". In the Gospel he had written, "In him [the Word] was life" (Jn 1:4) and elsewhere he records Jesus' statement, "I am the bread of life" (Jn 6:35, 48). These expressions declare that the Son of God has life in all its fullness, that is, divine life, the source of all life, natural and supernatural. Jesus in fact identified himself with Life (cf. Jn 11:25; 14:6). By the Incarnation, the Word of God manifests true life and at the same time makes it possible for that life to be communicated to men--imperfectly, by means of grace, while they are in this world, and perfectly in heaven, by means of the beatific vision (cf. 1 Jn 5: 12).
"And we testify to it": the testimony of the Apostles is something unique in the history of the Church, because (unlike those who succeed them) they know our Lord personally, they have been "witnesses" of his life, death and resurrection (cf. Lk 24:48; Acts 1:8).
"With the Father": the Greek implies closeness, difference, and the mutual relationship between Father and Son, so providing a glimpse of the mystery of the Blessed Trinity (cf. note on Jn 1:1).
3-4. This testimony about Christ is designed to lead to fellowship and complete joy. Fellowship with the Apostles (the Greek word is "koinonia") means, firstly, having the same faith as those who lived with Jesus: "They saw our Lord in the body," St Augustine reminds us, "and they heard words from his lips and have proclaimed them to us; we also have heard them, but we have not seen him [...]. They saw him, we do not see him, and yet we have fellowship with them, because we have the same faith" ("In Epist. Ioann. ad Parthos", 1, 3).
To have fellowship with the Father and the Son we need to have the same faith as the Apostles: "St John openly teaches that those who desire to partake of union with God must first partake of union with the Church, learn the same faith and benefit from the same sacraments as the Apostles received from the fullness of Truth made flesh" (St Bede, "In I Epist. S. Ioannis, ad loc."). The Church, the Second Vatican Council teaches, is not simply a collection of people who think the same way; it is the people of God "whom Christ established as a communion of life, love and truth" ("Lumen Gentium", 9).
Fellowship, communion, with the Apostles, with the Church, has as its purpose to bring about union with God ("with the Father and with his Son Jesus Christ"); this is a subject St John develops over the course of this letter, as he previously did in his Gospel (cf., e.g., Jn 17: 20ff). Here he uses expressions such as "to have the Son", and, in respect of the Son, "to have the Father" (2:23; 5:11ff); "to be in God" (2:5; 5:20); "to abide in God" (2:6, 24; 3:24; 4:13, 15, 16). This deep, intimate communion means that, without losing his personality, man shares in a wonderful and real way in the life of God himself. If Sacred Scripture uses many different expressions in this connection, it is due to the fact that the human mind, because it is so limited, cannot fully grasp the marvelous truth of communion with God.
Complete joy is the outcome of this communion. Most manuscripts say "our joy"; others, including the Vulgate, say "your joy". The difference is not important, because "our" involves the Apostles and the faithful, particularly in view of the mutual fellowship previously mentioned (cf. Jn 15:11; 17:13). This joy, which will reach its fullness in the next life, is already in this life in some sense complete, insofar as knowledge of Jesus is the only thing that can satisfy man's aspirations.
1:5-2:29. This section describes what communion with God is, and the demands it makes on us. We can say there are two parts in the section: the first (1:5 - 2: 11) teaches that communion with God means walking in the light and, therefore, rejecting sin and keeping the commandments. The second (2:12-19) warns the readers to guard against worldly concupiscence and not trust false teachers.
St John is writing as a pastor of souls who has lived the life of the Lord and reflected deeply upon it. His teaching interweaves truths of faith with moral and ascetical demands because he wants Christians to live in a way consistent with their faith. Therefore, the text does not really divide into a doctrinal section and a moral section.
Source: "The Navarre Bible: Text and Commentaries".