"My sinlessness protects me from all harm." (L337)
The sinlessness spoken of here is none other than the Self. The Course has been using the term "sinlessness" (noun) over the last few lessons. Here it is shown to be the Self, which is as God created It (unchanging, eternal, whole, at one with all creation etc.)
"My sinlessness ensures me perfect peace, eternal safety, everlasting love, freedom forever from all thought of loss; complete deliverance from suffering. And only happiness can be my state, for only happiness is given me. What must I do to know all this is mine?"
The Lesson then brings in a central idea running through the Course- that of the process of the Atonement.
"I must accept Atonement for myself, and nothing more. God has already done all things that need be done. And I must learn I need do nothing of myself, for I need but accept my Self, my sinlessness, created for me, now already mine, to feel God's Love protecting me from harm, to understand my Father loves His Son; to know I am the Son my Father loves."
The acceptance of something that already exists is not a 'doing' initiated 'of myself', but rather an allowing, opening up, and stopping to see what is actually present now.
The Course mentions that the Atonement is a process, which seems to take place in time. However, it is merely the recognition of what already Is. Accepting Atonement for one's self is thus nothing more than the recognition of What one truly is, the Self, ever present and without sin. Through the recognition of one's Self (sinlessness, purity, suchness), harm is impossible in whatever form, simply because What one truly is (which is prior to the body, mind, thought, feelings, etc.) cannot be harmed. Protection happens by the recognition that there is nothing threatened. Harm could only take place in the dream of death and guilt made by the ego, in which the Son equates himself with a body-mind bound burdened by debt, in time and space.
Now, what exactly is the Self? The Course leaves this answer in the hands of the Holy Spirit, God's Answer. Through practicing the Workbook, receptive listening, contemplation, and silence, the Self must reveal Itself, and the dream of the ego be seen through in its unreality (or as a mere concept). This becomes the "happy dream", where although the ego may still be in place and operative, it is known as just an habitual concept present in the mind of the Son of God, until it is no longer required.
Borrowing from other nondual teachings however, one can aid this process, in knowing the Self as one's own present, alive, conscious, beingness here and now. Self-attentiveness. That which is aware of the body, mind, thoughts, ego, and illusory identity within the dream. Becoming aquainted with this is the end and object of the whole Course, and will in time, and with 'a little willingness' and 'silence', be known through the grace of the Holy Spirit, which is none other than the Self.
"T-28.II.12. This world is full of miracles. 2 They stand in shining silence next to every dream of pain and suffering, of sin and guilt. 3 They are the dream's alternative, the choice to be the dreamer, rather than deny the active role in making up the dream. 4 They are the glad effects of taking back the consequence of sickness to its cause. 5 The body is released because the mind acknowledges "this is not done to me, but I am doing this." 6 And thus the mind is free to make another choice instead. 7 Beginning here, salvation will proceed to change the course of every step in the descent to separation, until all the steps have been retraced, the ladder gone, and all the dreaming of the world undone."