1 John 2:3-6

Verse 3

And hereby we do know that we know him - If we keep the commandments of God, loving him with all our heart, and our neighbor as ourselves, we have the fullest proof that we have the true saving knowledge of God and his Christ. The Gnostics pretended to much knowledge, but their knowledge left them in possession of all their bad passions and unholy habits; they, therefore, gave no proof that they had known either God or his Son Jesus - nor is any man properly acquainted with God, who is still under the power of his sins.
Verse 4

He that saith, I know him - This is a severe blow against those false teachers, and against all pretenders to religious knowledge, who live under the power of their sins; and against all Antinomians, and false boasters in the righteousness of Christ as a covering for their personal unholiness. They are all liars, and no truth of God is in them.
Verse 5

But whoso keepeth his word - Conscientiously observes his doctrine, the spirit and letter of the religion of Christ.

Is the love of God perfected - The design of God's love in sending Jesus Christ into the world to die for the sin of man τετελειωται, is accomplished, in that man who receives the doctrine, and applies for the salvation provided for him. This seems to be the meaning of the apostle.

That we are in him - That we have entered into his spirit and views, received his salvation, have been enabled to walk in the light, and have communion with him by the Holy Spirit.
Verse 6

Abideth in him - He who not only professes to have known Christ, but also that he has communion with him, and abides in his favor, should prove the truth of his profession by walking as Christ walked; living a life of devotion and obedience to God, and of benevolence and beneficence to his neighbor. Thus Christ walked; and he has left us an example that we should follow his steps.

To be in Christ, 1Jn 2:5, is to be converted to the Christian faith, and to have received the remission of sins. To abide in Christ, 1Jn 2:6, is to continue in that state of salvation, growing in grace, and in the knowledge of our Lord Jesus Christ.
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