The Apostle is writing to believers to reaffirm the Apostolic doctrine that Jesus is the Christ and the only access to God and eternal life (2.23; 5.12). He is proclaiming to them the truth about Christ handed down to him from the Father (1.2-5).
Abide in Him (2.28)
The readers are urged to abide in God through Christ—the only access to God. That is, they are to remain so close to Him as to naturally manifest His characteristics. The context suggests that abiding in God enables one to look forward with confidence to the second coming.
Help the Brethren (3.17-18)
Since God willingly sacrificed His Son for believers, the readers are to properly respond to that love by helping one another sacrificially. Their love for one another should not be in mere words, but should be apparent to all who look on (3.18).
Love one another (3.11; 4.7, 20-21)
Again, in light of God’s sacrificial love for believers in sending His Son, the readers are to reflect that same love in their relationships.
Do not love/marvel at the world (2.15; 3.13)
Since the whole world is under the power of the evil one (5.19), the readers should refrain from loving it or the things in it (2.15). Furthermore, since the world hated the Savior, the readers should not be surprised if the world likewise hates them (3.13).
Do not let them deceive you (3.7)
The writer is attempting to prevent the reader from being influenced by the antagonists and buying into their false doctrine that the Father can be accessed apart from Christ.
The author is inviting the reader to accept his theology—the Apostolic doctrine—rather than the antagonist’s errant one.
The revelation of the Apostles and the Old Testament provide the substance of the author’s doctrines. This Apostle had a clear understanding of the Old Testament, seen not only from his explicit references (3.12), but also from his use of it as a foundation for his letter (1.5-3.24).
The Apostle was teaching that one has access to the Father through Christ alone (2.22-23; 5.11-13). Good works cannot provide access to the Father, nor procure eternal life, since sin still exists (3.1-24). When one trusts in Christ, however, he has an Advocate with the Father who has cleansed him from his sins (1.7; 2.10). Additionally, eternal life is secured only through the Son (5.11-13), and one who trusts in Him can have confidence at His coming (4.17).
When one acknowledges Jesus as the Christ, realizing the sacrifice incurred by the Father, he responds with reciprocal love for his God, and sacrificial love for his brother (3.23).
The Apostle is writing to believers to reaffirm his theology that; The Old Testament and Apostolic Revelation received from Christ indicate that Jesus Christ is the only access to the Father and eternal life, which will result in belief in Him and a response of love for one another.