The Creed of Nicaea starts with a declaration of belief "in one God, the Father, Almighty, maker of all things visible and invisible" then expresses belief in the one Lord, Jesus Christ [see 1 Corinthians 8:6] before stating belief in the Holy Spirit.
Of Christ, the Creed says that he is "only-begotten, that is, from the substance of the Father: God from God, Light from Light, Very God from Very God, begotten not made, of one substance with the Father, through whom all things were made, both in heaven and on earth..." and so on.
So far, there is nothing that even hints at doubt as to the eternality of the Son.
However, you say that being 'of or from God indicates a beginning', which may show a lack of understand of what the Creed means. This may be due to having a modern, English-language view of what 'only-begotten' means, and fails to grasp the koine Greek words used in the Bible that deal with this. Rather than delve into details about that, let me quote this book to show the problems being dealt with, leading up to the Creed using those phrases.
"To exclude Arian error, the Council produced its own creed [which I
give some details of above]...
Apparently Arius could agree to any statement using solely biblical
language. Constantine supported the introduction of the word
'consubstantial' - probably suggested by a Western bishop.
'Consubstantial' (homoousios) had been introduced to Christian
theology by Gnostics who believed that the heavenly powers shared in the
divine fullness. Similarly Origen probably applied it to the Son as a
true offspring of the Father, but later bishops had been unhappy about
its implications. For many at Nicaea it probably implied that the
Son was no less divine than the Father; that the two were equally
divine, as an earthly father and son are equally human. For the
Westerners and a few Easterners, Alexander and Athanasius, his
personal assistant, Eustathius of Antioch and Marcellus of Ancyra, it
meant that Father and Son were one in a single Godhead. Both
these senses ruled out Arian misconceptions..." The History of
Christianity, pp.158-9, Lion, 1985
Now, seeing the background to this Creed, it can be understood why it then went on to state:
"And those who say: 'There was a time when he was not', and:
'Before he was begotten he was not', and: 'He came into being from
nothing', or those who pretend that the Son of God is 'Of another
substance, or essence', or 'created' or 'alterable' or
'mutable', the catholic and apostolic church places under a curse."
(Ibid. p159) Emphasis mine
Because of all the semantics tricks played by Arius and those sympathetic to him, that creed had to make those detailed statements, followed with anathemas against those who believed as Arius believed. Arius believed there was a time when the Son did not exist, being created at a point in time by the Father, and that he was actually an angel.
Now that this has been made clear, it can be seen that the Creed of Nicaea is diametrically opposed to any notion of the Son of God (also known as the Word of God, John 1:1-14) not being eternal, as the Father is eternal. A main purpose of the Creed was to make it impossible for those (like Arius) who did believe the Son was created, that the Son was not eternal, to hold any office in the Christian church.