The Nicene Creed of 325 AD begins with a section which says:
The Nicene Creed (325 AD)
We believe in one God, the Father almighty, Maker of heaven and earth, and of all things visible and invisible. And in one Lord Jesus Christ, the Son of God, the only-begotten, begotten of the Father before all ages. Light of Light, true God of true God, begotten not made, of one essence with the Father by whom all things were made;
The above describes Jesus as "true God from true God". Therefore, according to this creed, there would be two who can be called "true God", the Father, and Jesus.
It is worth noting that this directly contradicts the statement of Jesus Christ in John 17:3, wherein He explicitly distinguishes Himself from "the only true God", who is the Father:
John 17:3 (WEB)
3 This is eternal life, that they should know you, the only true God, and him whom you sent, Jesus Christ.
Would anyone, hearing the above from the mouth of Jesus in the first century, conclude that He Himself is "the only true God"? Jesus Christ is speaking, and He says that "the only true God" has sent Him, and in doing so, refers to "the only true God" as a separate subject from Himself. Given this, if Jesus were in fact "the only true God", He would have necessarily sent Himself, something He denies elsewhere in the Gospel of John (John 7:28-29, 8:42).
Therefore, a straightforward reading of John 17:3 establishes that the Father is "the only true God", and that means that no one else is "true God". So, the Nicene Creed's designation of Jesus as "true God from true God" is directly contrary to the words of Jesus in the Gospel of John.
Some may respond that Jesus is Himself called "true God" in 1 John 5:20. However, close examination of that passage demonstrates that this is a misreading, and that the Father is being referred to there as well.
Others may respond that the Trinity teaches that all three Persons are one Being, which is the "true God". Therefore, any of them can say of each other that they are the "only true God", because they comprise one instance of the divine Essence.
In response, consider the passage again. Jesus refers to the Father as "the only true God". But, if the only true God is actually the Father, Jesus Himself, and the Holy Spirit, then the Father would not be "the only true God", because the Spirit and Son also comprise "the only true God". So, only all three of them as a group could rightly be called "the only true God". Additionally, this reading would have Jesus saying, in effect, "This is eternal life, that they may know you, the only true God, and (myself, the only true God) who you have sent", which turns a rather simple statement into a very complex, and apparently contradictory one. Finally, as mentioned before, Jesus denies sending Himself (John 7:28-29, 8:42), whereas this passage teaches that "the only true God" sent Him.
Another response to this critique may be to suggest that "the only true God" is a qualitative rather than numerical designation. In other words, Jesus is saying that the Father has the divine Essence, which itself can be said to have "only true Godness", for lack of a better term. Therefore, this is a statement about the Essence being the "only" true divine Essence, and not that God the Father is the "only" one who has it.
In response, notice that the statement is not addressed to an Essence, but explicitly to a Person. The passage in question is part of a prayer addressed to the "Father" (John 17:1), and is speaking to Him. And, this "only true God" "sent" someone, which is a personal action, not a trait of an impersonal Essence.
Finally, consider again the apparent, or "naïve" reading of the passage. It would be very difficult to argue that this passage does not at least appear to be explicitly Unitarian, even if one were to believe in the Trinity. Taken simply, it is Jesus praying to the Father, calling Him "the only true God", and explicitly distinguishing Himself from that figure. The reading of this passage under a Trinitarian system is comparatively very complex, and unintuitive.
The Nicene Creed's "true God from true God" statement is apparently contradictory to monotheism, teaching two who are "true God". In Scripture, Jesus calls the Father "the only true God". The Nicene formulations required in attempt to reconcile the Creed's statement are extremely complex, and require a very unintuitive reading of what is a rather straightforward passage of Scripture, John 17:3.