In his greeting to the congregation the Elder repeats two important themes: truth (vv. 1-4) and love (vv. 1, 3, 5-6). Truth includes matters of both faith and practice, and thus designates what Christians are to believe (vs. 7; 1 Jn 4:2; 5:6) and how they are to live (vv. 5-6). Truth is the reality to which Christians are committed, and they are known by their commitment to it.
But that reality is not simply a static and objective entity or set of beliefs. We tend to think of truth as a number of abstract propositions that we are to comprehend and believe. But for the Elder, truth is a vital force that can be personified as living in us and being with us. Because it comes from the living God, truth is a dynamic power that abides with believers, enabling them to know what is true. And because truth comes from God, it exists forever and remains with the faithful, just as God exists eternally and remains in relationship with the faithful. If we could capture John's view of truth as a force that, because it is the work of God's own Spirit, shapes and empowers us, we might be less prone to think of truth as something that depends on us to preserve it. In reality, we depend upon the truth to guard us - an not vice versa - because we depend upon God. Only as the truth abides in us do we abide in the truth. But we are somewhat too quick to reverse that relationship, and put human beings in the place where God's activity and power belong.
- Marianne Meye Thompson 1-3 John (IVP New Testament Commentary Series) pp. 151-2.