"It should especially be noted that God has only one son, yet ALL HIS CREATIONS ARE HIS SONS, every one must be an integral part of the whole Sonship. The Sonship in its oneness transcends the sum of its parts (T-2.VII.6:1-3)."
Although it leads by saying God has one son. It follows by telling us that God has MANY creations. And that therefore, it logically follows, that ALL of those creations must be PART OF the one son. ... that ALL of God's creations have the same characteristics, being made in HIS LIKENESS. They all share the same holiness, the same innocence, the same mentality, the same love. Yet there are many.
And because they are SO ALIKE AS TO BE IDENTICAL, yet still MANY... all of them together are "the sonship", and because oneness transcends separation the sonship as a whole is more than the sum of its parts. God + sons + angels + heaven + whatever else is all one big oneness, yet still SOMEHOW has multiple parts to it.
"WE are creation; WE the SONS OF GOD. We seem to be discrete, and unaware of our eternal unity with Him. Yet back of all our doubts, past all our fears, there still is certainty. For love remains with all its Thoughts, its sureness being theirs. God's memory is in OUR (many) HOLY MINDS, which know their oneness and their unity with their Creator (W-pII.11.4:1-5)."
CLEARLY... "oneness" does not mean "just one by counting". We do not understand oneness. Oneness means SHARING AND EXTENSION. Oneness must be shared, it must extend, it must CREATE. This is how even "the one son of God" was created in the first place, because truly if there was ONLY ONE ONENESS, there would ONLY BE GOD, and there WOULD NOT BE A SON!
Oneness creates by extension of itself, and must extend in order to exist. Therefore, it HAS to create a son, and the son has his own creations, and God HAS to create many sons. Yet they are one.
When our EGO tries to understand how this is possible it becomes confused and it makes no sense, because THE EGO CANNOT UNDERSTAND GOD. It is simply something we have to accep is true, and TRUST that because Jesus has shared this information, IT MUST BE TRUE.
There are many sons of God, there are many minds that God created, ALL of them together is "the son ship" or what, TO GOD, is experienced as "ONE SON".... referring to the grand inclusive "experiences them all at once" kind of experience, knowing their equality and sameness, as though they are all identical, AND YET... He has created MANY BY NUMBER, somehow. In a way that we cannot possibly understand or get our head around, because numbers imply separation and bla bla bla bla bla.... NEVER will get this. It has to just be accepted as true, trusting that Jesus is telling us the truth.