1 John 4:14-15

And we have seen and testify that the Father has sent his Son to be the Savior of the world.

(v14, NIV)

If you recall from the beginning of 1 John, John is speaking to a group of believers who have actually seen and heard and testify to the physical living Christ as he walked this earth. We have received his Spirit and have seen and testified to the Son whom the Father sent to be the savior of the world. What more proof do you need? Yet, some will be presented with such proof and not be swayed. Regardless, these are people who have seen and testified and are assured by a number of different things.

If anyone acknowledges that Jesus is the Son of God, God lives in him and he in God.

(v15, NIV)

John follows with an assurance of those who acknowledge that Jesus is the son of God. Not only do these people have his Spirit, but they have seen and testify to Jesus who is the savior of the world. These things are evidences of certain people. These certain people are those whom God lives in and who live in God as well. God in you and you in him. An outpouring of his Spirit and testifying to the Son is what these people do.