Clarification of Terms: Mind - Spirit
The term mind is used to represent the activating agent of spirit. The term spirit is the Thought of God which He created like Himself, which includes the part of you that is still in contact with God through the Holy Spirit. You are, then, in truth -- spirit, a Thought of God. The mind supplies the creative energy to spirit. As you think, so do you perceive yourself. If you think of yourself correctly, as spirit, you will perceive yourself correctly and activate spirit. If you think of yourself incorrectly, as body/ego/etc., then you will perceive yourself incorrectly and will activate the ego. The mind does the choosing of which to activate: spirit or ego.
In this world, because the mind appears split between truth and deception (spirit and ego), the Sons of God appear to be separate; their minds do not seem to be joined in truth. Therefore there appears to be much discord and conflict in the world. Peace and harmony are experienced only when minds are joined in thought, and they can only join when thinking the truth. If minds agree on what is NOT true, there is still no "joining" because minds cannot join in deception or illusion, only in TRUTH. In this illusory idea of a split or dual mind, the concept of an "individual mind" therefore seems to be meaningful. The mind is therefore described in the course as if it had two parts: spirit and ego.
The unified spirit is God's one Son, or Christ. Spirit is the part of you that is still in contact with God through the Holy Spirit, Who abides in this part, but still sees the part you made as well. So the Holy Spirit can help you to think correctly, using right-mindedness to activate spirit (instead of wrong-mindedness to activate the ego). As you think with Him, and as your brothers think with Him, you think together. And where two or more are gathered together thinking only Thoughts of God, there is a joining of mind and spirit with each other and with Him. There is unification.
There is power in this joining, power to overcome or overlook the world that was made to keep you separate from each other and separate from your Father. Power is in spirit, because there is power in truth. There is an illusion of power in the ego, but it is not real and is like dust in the wind that is easily blown away simply by again choosing to activate spirit. Spirit and ego will not be activated at the same time. It is one or the other, not both (and though it is not recommended, it is possible to vacillate between the two). The ego part of the mind is entirely illusory and makes only illusions. Spirit retains the potential for creating, but its Will, which is God's, seems imprisoned while the mind is not unified, while it still seems split between spirit and ego.
God's Will is always free for us to choose in this world. In fact, it is the only remaining freedom. The choice is between two voices: the Voice for God or the voice(s) of the ego. Consciousness is the receptive mechanism, receiving messages from above or below, from the Holy Spirit or the ego. Ultimately God's Will is the ONLY choice -- because we share His Will for us. The ego is unreal and will ultimately be seen as such.
The mind can be right or wrong, depending on the Voice/voice to which it listens. Right-mindedness listens to the Holy Spirit, the one Voice for God, and forgives (overlooks) the errors of the world. These errors never happened in truth, as they are all illusions and misperceptions fostered by the ego. Through Christ's vision, the right mind sees the real world in place of the appearance of error -- the real world where time and place have an end, and the truth of timelessness and spacelessness reign in light.
Wrong-mindedness listens to the many voices of ego and makes illusions seem real, perceiving sin, justifying anger, and seeing guilt, disease and death as real. Both the ego world and the real world are illusions, however. Heaven, our natural environment, is the truth.
Using the right mind to forgive or to overlook illusions, is still not the same as being awake to the truth; not the same as being in Heaven. To enter Heaven, then, the mind cannot simply be unified. It must be the One-mindedness of the Christ Mind, Whose Will is one with God's. God Himself will lift you into Heaven when One-mindedness is reached. The state of grace is reached first, and then God Himself will take the last step for you, lifting you into Heaven, one with His Love.
Consciousness has levels and awareness can shift quite dramatically from truth to illusion and back again; but it can never transcend the perceptual realm, and it will never become knowledge. Knowledge is of God and Spirit, and only when One-mindedness is reached will knowledge be known. Consciousness, at its highest level, becomes aware of the real world, and can be trained to do so increasingly. But because it has levels and can be trained (this is a mind-training course), it cannot reach knowledge. Knowledge is given, not learned.
To summarize, then, the mind is the activating agent of spirit. In this world you can choose to activate spirit, and leave the ego to disappear into the nothingness from which it came. This will enable your mind to remain open and receptive for the Holy Spirit to guide you to overlook error and see truth. Truth is seen in light. Light is seen in the real world. So ask to see the real world and see it more and more, ask for God's grace, ask and allow the Holy Spirit to prepare you for your real home in Heaven where GOD IS.