"This writing begins without any of the formal features characteristic of a letter, such as we found in2John and3John. Since the conclusion also lacks any typical features of a letter, we must conclude that the writing is not so much a letter as a written sermon or address." [1][source]
John began this epistle by explaining to his audience why he wrote. He said he wrote so his readers would enjoy the fellowship with God that is possible only to those who have seen Him. This fellowship, he explained, rests on the reality of Jesus Christ"s incarnation, and it results in full joy for those who experience it. [source][source][source]
"No writer in the New Testament holds with greater intensity the full reality of the incarnation." [2][source]