The Father and Son are. The Holy Spirit is a spirit.
According to Greek Grammar he used it as one in essence.
John 10:30 I and my Father are one.
John 10:30
Egoó kaí ho Pateér hén esmen
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I and my Father one are
(Interlinear Transliterated Bible. Copyright (c) 1994 by Biblesoft)
"One. Gr. "hen" Neut., one in essence,
not one person which would be "heis", masc. This is the climax of His claim to oneness with The Father in vv. 18, 25, 28, 29. Compare also V. 38; 14:11 Rev 22:3"
According to Bullinger, an accomplished Greek scholar, the word for "one" is "hen" the Neut. form of the word "heis". "heis" means "one" of person but "hen" means "one" in essence and not one person!
John 10:30 I and my Father are one [in essence].
or
John 10:30 I and my Father are [spiritually] one.
This is what Christ said.
Matthew Henry also confirms this:
http://www.ccel.org/h/henry/mhc2/MHC43010.HTM
Further to corroborate the security, that the sheep of Christ may have strong consolation, he asserts the union of these two undertakers: "I and my Father are one, and have jointly and severally undertaken for the protection of the saints and their perfection." This denotes more than the harmony, and consent, and good understanding, that were between the Father and the Son in the work of man's redemption. Every good man is so far one with God as to concur with him; therefore it must be meant of the oneness of the nature of Father and Son, that they are the same in substance, and equal in power and glory. The fathers urged this both against the Sabellians, to prove the distinction and plurality of the persons, that the Father and the Son are two, and against the Arians, to prove the unity of the nature, that these two are one. If we should altogether hold our peace concerning this sense of the words, even the stones which the Jews took up to cast at him would speak it out, for the Jews understood him as hereby making himself God (v. 33) and he did not deny it. He proves that none could pluck them out of his hand because they could not pluck them out of the Father's hand, which had not been a conclusive argument if the Son had not had the same almighty power with the Father, and consequently been one with him in essence and operation.
"the union of these two undertakers"
"therefore it must be meant of the oneness of the nature of Father and Son"
"to prove the distinction and plurality of the persons, that
the Father and the Son are two"
"one with him in essence and operation"
Matthew Henry also knew that the greek for "one" was meaning one in essence, not in person as Bullinger also confirmed.
Gill
I and my Father are one. Not in person, for the Father must be a distinct person from the Son, and the Son a distinct person from the Father; and which is further manifest, from the use of the verb plural, "I and my Father", esµe?, "we are one"; that is, in nature and essence, and perfections, particularly in power
Robertson's word pictures:
John 10:30
One (hen). Neuter, not masculine (heis).
Not one person (cf. heis in Gal_3:28), but one essence or nature.