JMNT(i)
10 The person continuously and progressively believing (or: keeping confidence and habitually putting trust) into the midst of God's Son constantly holds (or: has; possesses) the testimony (witness; evidence) [p74 & A add: of God] within himself; the one not believing in God [A reads: the Son] has made Him out to be (or: has construed Him) a liar, because he has not believed or put trust into the evidence (testimony; witness) which God has attested and affirmed concerning His Son (or: shown as proof round about the Son from, and which is, Him).
11 And so this is the evidence (or: exists being the testimony, witness and attested affirmation): that God gives (or: gave; grants) Life pertaining to, and having the quality of the Age (life whose source is the Age [of Messiah]; eonian life; Life of, for and on through the ages) to, for and in us, and this very Life continuously exists within His Son (or: is in union with the Son which is Him)!
12 The one continually holding (or: constantly having; progressively possessing) the Son continuously holds (constantly has; progressively possesses) the Life. The person not continuously holding (constantly having) God's Son does not now have (or: hold) the Life. [comment: it seems that the Son is the Life]
13 I write these things to you folks to the end that, having seen, you may know that you are presently holding (you folks constantly and continuously have) Life pertaining to, and having the qualities of, the Age (life whose source is the Age [of Messiah]; or: eonian life; Life of, for and on through the ages) – for the folks (or: in and among the ones) continuously believing and putting their trust, confidence and reliance into the Name of God's Son.