Read 1 John 1:1-4
There are three key words in the epistle of 1 John. They are "life," "light" and "love." These words are used many times in this epistle.
That God has given us eternal life is clearly stated in 1 John 5:11-13, which are key verses to the study of this epistle. God has given us eternal life, and that life is in His Son. If we have the Son, we have eternal life; if we do not have the Son, we do not have eternal life.
Eternal life is a special quality of life that makes it possible for us to fellowship with God. More than eternal duration of life is meant.
The other two words, "light" and "love," provide the test whereby we may know whether or not that life is within us.
We are told that God, as to His nature, is light and that there is no darkness in Him. And if we walk in the light as He is in the light, we have fellowship one with the other.
Light reveals righteousness, and it also reveals sin. By virtue of the light, we can know whether or not we have eternal life.
The love spoken of is a God-given love, and it becomes both the outward expression of the inward life and another test whereby we may know that we have such life.
The love of God has been shed abroad in our hearts by the Holy Spirit (Rom. 5:5) so that the man who is born of God loves others. This is the very essence of Christianity.
"Eye hath not seen, nor ear heard, neither have entered into the heart of man, the things which God hath prepared for them that love him" (1 Cor. 2:9).