Of the seventy times in which the demonstrative pronoun ουτος has a personal referent in the Gospel of John and his Epistles, the Father is never the referent! This fact, along with proximity, significantly increases the likelihood that Jesus Christ is the antecedent in the case of 1 John 5.20, as thus…
οιδαμεν δε οτι ο υιος του θεου ηκει και δεδωκεν ημιν διανοιαν ινα γινωσκομεν τον αληθινον και εσμεν εν τω αληθινω εν τω υιω αυτου ιησου χριστω ουτος εστιν ο αληθινος θεος και ζωη αιωνιος
oidamen de hoti ho huios tou theou hēkeikaidedōkenhēmindianoianhinaginōskōmentonalēthinonkaiesmenentō huiō autouIēsouChristō houtosestinhoalēthinostheoskaizōē aiōnios
And we know that the Son of God has come, and He has given to us an understanding that we may know the true One, and we are in the true One, in His Son Jesus Christ. This is the true God and the life eternal. (1 John 5.20)
Regarding John 17.3, if you were familiar with Greek, then you would already be cognizant that there are absolutely no grammatical reasons at all for denying that αληθινον θεονrefers to Jesus Christ.
This can be deduced from a study of the article with multiple substantives connected via kai.
αυτη δε εστιν η αιωνιος ζωη ινα γινωσκωσιν σε τον μονον αληθινον θεον και ον απεστειλας ιησουν χριστον
hautē de estin hē aiōnios zōē hina ginōskōsin se ton monon alēthinon theon kai hon apesteilas Iēsoun christon
And this is eternal life, that they may know You, the only true God, and Jesus Christ, whom You have sent. (John 17.3)
οιδαμεν δε οτι ο υιος του θεου ηκει και δεδωκεν ημιν διανοιαν ινα γινωσκομεν τον αληθινον και εσμεν εν τω αληθινω εν τω υιω αυτου ιησου χριστω ουτος εστιν ο αληθινος θεος και ζωη αιωνιος
oidamen de hoti ho huios tou theou hēkeikaidedōkenhēmindianoianhinaginōskōmentonalēthinonkaiesmenentō huiō autouIēsouChristō houtosestinhoalēthinostheoskaizōē aiōnios
And we know that the Son of God has come, and He has given to us an understanding that we may know the true One, and we are in the true One, in His Son Jesus Christ. This is the true God and the life eternal. (1 John 5.20)
Regarding John 17.3, if you were familiar with Greek, then you would already be cognizant that there are absolutely no grammatical reasons at all for denying that αληθινον θεονrefers to Jesus Christ.
This can be deduced from a study of the article with multiple substantives connected via kai.
αυτη δε εστιν η αιωνιος ζωη ινα γινωσκωσιν σε τον μονον αληθινον θεον και ον απεστειλας ιησουν χριστον
hautē de estin hē aiōnios zōē hina ginōskōsin se ton monon alēthinon theon kai hon apesteilas Iēsoun christon
And this is eternal life, that they may know You, the only true God, and Jesus Christ, whom You have sent. (John 17.3)