Let’s start here, in 1 John 3:2:

Beloved, now we are children of God;

and it has not yet been revealed what we shall be,

but we know that when He is revealed,

we shall be like Him,

for we shall see Him as He is.

The Revelation of Jesus Christ,

which God gave Him to show to His bond-servants,

the things which must soon take place;

and He sent

and communicated it by His angel to His bond-servant John,

who testified to the word of God

and to the testimony of Jesus Christ,

even to all that he saw.

The elder to the beloved Gaius, whom I love in truth.

The apostle uses the same term for himself as he did in the second epistle—elder.

The recipient of John’s third letter is the head of a church under John’s care, and he wants Gaius to know just how much he is appreciated and loved for his faithfulness.

The elder to the chosen lady and her children,

whom I love in truth;

and not only I,

but also all who know the truth,

for the sake of the truth which abides in us

and will be with us forever:

Grace, mercy and peace will be with us,

from God the Father

and from Jesus Christ,

the Son of the Father,

in truth and love.

These things I have written to you

who believe in the name of the Son of God,

so that you may know that you have eternal life.

As John brings to a close his letter to the churches in Asia, he explains why it was written in the first place.

If we receive the testimony of men,

the testimony of God is greater;

for the testimony of God is this,

that He has testified concerning His Son.

This is the One who came by water and blood, Jesus Christ;

not with the water only,

but with the water

and with the blood.

It is the Spirit who testifies,

because the Spirit is the truth.

Whoever believes that Jesus is the Christ is born of God,

and whoever loves the Father loves the child born of Him.

Whoever confesses that Jesus is the Son of God,

God abides in him, and he in God.

In one sentence, John implores his reader to believe in the Trinity.

Beloved, let us love one another, 

for love is from God;

and everyone who loves is born of God

and knows God.