That they may know Thee, the only true God. Hence the Arians infer that Christ is not true God. In reply,
S. Augustine (in loc.), Bede, and others, connect together Jesus Christ and the Father under the one term “Deity,” and interpret thus, As the Father is true God, so is the Son also true God. (SeeS. Hil. lib. ix. de Isaiah.) The statement would otherwise be imperfect, for if we believed that the Father alone was true God, we could not have anything else to say about Jesus Christ, unless we understood that He was true God also. The Fathers, in fact, infer from this Christ’s Godhead.
S. Chrysostom, Cyril, and others reply that the word “only” does not exclude the Son and the Holy Spirit, but merely idols and false gods. And the meaning is that they may believe in Thee, who art that God, who only is the true God, as is also the Son and the Holy Spirit. That the Son is true God is sufficiently indicated, when it is said that eternal life consists in the knowledge of Him and of the Father alike. For eternal life necessarily consists in (the knowledge of) the one supreme and true God. (See S. Ambrosede Fide, v. 2.) Christ therefore through modesty does not call Himself God, but one sent by the Father, as the Redeemer of the world. For such He was when Incarnate, and made man. And hence we infer that faith in the Incarnation and the Trinity is required in order to salvation. For the Father cannot be fully believed in, apart from the Son and the Holy Spirit, for the Paternity of the Father requires also the breathing forth of the Holy Spirit.