And this is the promise which Christ [or, "God"] has given to us: [It is] never ending life. [Note: This passage may mean that God promised us "the Eternal Life," that is, Christ, See John 3:16].

This is [one way] to know we live in [fellowship with] God and that He lives in our hearts: [It is] because He has given us His Holy Spirit [to live within us, See 3:24].

And [in addition], we know that the Son of God has come [in a physical body, See 4:2] and has given us understanding, so that we can know the True One [i.e., God], and that we are in [fellowship with] the True One, even His Son Jesus Christ. This Son is the true God and eternal life.