KalebOutWest : The verse [Revelation 3:14] has not been altered. We have the original reading.
As the OP is referring to the codex Sinaiticus, it is quite clear the verse has been altered. I show below the Greek and English text of Nestle-Aland 28 and codex Sinaiticus for comparison.
Revelation 3:14
Sinaiticus
και τω αγγελω τηϲ εν λαοδικια εκκληϲιαϲ γραψο ταδε λεγει hο αμη και hο μαρτυϲ hο πιϲτοϲ και hο αληθινοϲ και hη αρχη τηϲ εκκληϲιαϲ του θυ
And to the angel of the congregation in Laodicea write: These things says the Amen, and the faithful and true witness, and the beginning of the church of God;
NA28
και τω αγγελω τηϲ εν λαοδικεια εκκληϲιαϲ γραψον ταδε λεγει hο αμην hο μαρτυϲ hο πιϲτοϲ και αληθινοϲ hη αρχη τηϲ κτιϲεωϲ του θεου
And to the angel of the congregation in Laodicea write: These things says the Amen, the faithful and true witness, the beginning of the creation by God;
Whether the "original" (by which I presume you mean NA28 or ECM) reading was harmed or not, is not the point. The point is why the copyist of the fourth century manuscript changed the text from "the beginning of the creation by God" to "the beginning of the church of God". This cannot be explained by a slip of the pen or skipping a line whilst copying. It is a deliberate alteration of the text. You can see in the Greek text there is no similarity between κτιϲεωϲ and εκκληϲιαϲ. And the noteworthy thing is that the correctors (of which there were several) failed to correct it for three hundred years, that is until the seventh century.
KalebOutWest : Besides, the Trinity dogma is not based on this text whatsoever. The dogma was set in 325 CE at the Council of Nicea.
Of course, the Trinity dogma is not based on this text as it offers no support for it. On the contrary. But the fact that Arius asserted that the Son was the beginning of all creatures, using the same wording as Revelation 3:14, indicates that he was familiar with the expression.
Revelation was known and used long before the Council of Nicaea. Justin Martyr (c.100-165) shows a clear knowledge of it in his discussion of the ‘Millenium of Peace’ (see his Dialogue with Trypho 81.4). The Epistle of Vienne and Lyons, relating to the persecution of Christians in Gaul about 177, quotes from Revelation 22. Also, parts of it are found in several papyri prior to the fourth century: P. Chester Beatty 3 (P47) originally contained the whole of Revelation and dates to the late 3rd century. P98, containing parts of the first chapter, is dated to the late 2nd or early 3rd century. The book must obviously have been recognised some time before then to end up in Egypt in the second/third century.
Whether or not it was included in some official canon is irrelevant. Quite clearly the copyist who wrote codex Sinaiticus thought it important enough to include it and considered 3:14 to be influential enough to have to change it.