This is the testimony, that God …: the demonstrative pronoun is pointing forward to the that clause, which is not stating the contents of the testimony but its effect. To bring this out one may have to say ‘what the testimony leads to is that God…,’ ‘the testimony means/implies that God….’

The testimony refers to “the testimony God has borne to his Son” (verse 10b). To bring this out one may use ‘God’s testimony.’ But in some cases the repetition of this noun sounds unduly redundant, and it is preferable to substitute another noun or a pronoun; for example, ‘these words of God mean that he…,’ or ‘the essence of it is: God…’ (Bijbel in Gewone Taal).

That God gave us eternal life, or, to bring out the emphatic position eternal life has in the Greek, ‘that it is eternal life which God gave.’ If the noun is to be rendered as a verb, one may have to shift to something like ‘that God causes us to live fully/truly/eternally.’ For eternal life see comments on 1.1-2.

The verb gave is in the aorist, referring to something that happened once, namely, at the coming of Jesus Christ.

This life is in his Son: the clause may or may not be dependent on “that.” For the thought expressed in this and the preceding clause, compare also John 3.15-16.

The preposition in serves to show that the eternal life is closely bound to, and offered through, the Son of God. Accordingly a possible restructuring is ‘this life he gave us through his Son.’ For a similar use of the preposition in, compare John 16.33, “that in me you may have peace.”

Quoted with permission from Haas, C., de Jonge, M. and Swellengrebel, J.L. A Handbook on The First Letter of John. (UBS Handbook Series). New York: UBS, 1972. For this and other handbooks for translators see here .