Jesus The Author - How Can We Be Sure?

On what basis can one accept that the "Voice" Helen Schucman heard was that of Jesus? Each person must decide whether to accept that or not. However, there are many passages throughout A Course in Miracles (ACIM) which show he is the author; for instance, this passage from the Text.

I have been correctly referred to as "the lamb of God who taketh away the sins of the world," but those who represent the lamb as blood-stained do not understand the meaning of the symbol. Correctly understood, it is a very simple symbol that speaks of my innocence.
[T-II:5.1-2]

Here is another such passage.

Do not make the pathetic error of "clinging to the old rugged cross." The only message of the crucifixion is that you can overcome the cross. Until then you are free to crucify yourself as often as you choose. This is not the Gospel I intended to offer you. We have another journey to undertake, and if you will read these lessons carefully they will help prepare you to undertake it.
[T-4.in.III:7-11]

Yet, the Jesus speaking to us in A Course In Miracles (ACIM), is very different from the Historical or Biblical Jesus. In this excerpt from Clarification of Terms that is evident.

The name of Jesus is the name of one who was a man but saw the face of Christ in all his brothers and remembered God. So he became identified with Christ, a man no longer, but at one with God. The man was an illusion, for he seemed to be a separate being, walking by himself, within a body that appeared to hold his self from Self, as all illusions do. Yet who can save unless he sees illusions and then identifies them as what they are? Jesus remains a savior because he saw the false without accepting it as true. And Christ needed his form that he might appear to men and save them from their own illusions.

In this next section the "Christ," as taught in A Course In Miracles (ACIM), is defined.

In his complete identification with the Christ—the perfect Son of God, His one creation and His happiness, forever like Himself and one with Him---Jesus became what all of you must be.

And further:

Is he the Christ? O yes, along with you. His little life on earth was not enough to teach the mighty lesson he learned for all of you. He will remain with you to lead you from the hell you made to God. And when you join your will with his, your sight will be his vision, for the eyes of Christ are shared. [C-5.II:1-6]

The above excerpts present two basic concepts that make A Course in Miracles (ACIM) a unique spiritual path. One, we have the capacity to be as Jesus is; and two, "Christ" is the Perfect Son of God in which we are all joined in Heaven, along with Jesus.