Like Peter, we could be motivated to believe or disbelieve something, accept or reject something, say something or keep silent, depending on the circumstance. Additionally, we may have no reaction at all at the moment of communication, but the thought is stored and available for later use or supplementation. It is entirely possible for a person to go through his entire life as a pawn of Satan and never know it.
This situation reflects a usage of what the Bible's writers term "spirit." Spirit is the English translation of the Hebrew ruach (Strong's #7304), in the Old Testament and the Greek pneuma (Strong's #5141) in the New. It can literally mean "a current of air," "breath," "blast," or "breeze." However, when used figuratively, it indicates "vital principle," "disposition," "the rational soul," etc., or an invisible super-being such as God, Christ, an angel, or a demon. Whether used literally, as with "wind" or "breath," or figuratively, as indicating God, angel, or demon, it describes something that is invisible and immaterial and at the same time powerful, even a thing of considerable power. The foremost elements of spirit, then, are invisibility, immateriality, and power.
E.W. Bullinger remarks in Appendix 9 of the Companion Bible:
The meaning of the word is to be deduced only from its usage. The one root idea running through all of the passages is invisible force. . . . [I]n whatever sense it is used, [it] always represents that which is invisible except by its manifestations.
He also shows that ruach is used in nine different ways in the Old Testament, while pneuma is used fourteen different ways in the New Testament.
In John 6:63, Jesus says, "It is the Spirit [which] gives life; the flesh profits nothing. The words that I speak to you are spirit, and they are life." Here is a clear example of the figurative use of "spirit." Words are the symbols used for communication; they are received into our minds through sight, as when reading, or sound, as when hearing. But once in the mind, nothing material is packed into our brain. Words - and thus the concepts they carry with them - are spirit because they are immaterial, invisible, and of considerable power, depending on how we use them. Thus, we can receive "spirit" in the form of words or concepts from a spirit being. In this case, it is in reality "thought transference" because no sound is heard through our ears.
Just because one is close to Christ does not eliminate the prospect that a demon will communicate with and through him. As seen in Matthew 16:22-23, Peter did the speaking, but Jesus spoke directly to Satan, naming him as the source of Peter's outburst against God's will that Jesus should suffer and die. Without Peter's recognizing it, he permitted himself to be a conduit for Satan's will. The disciple's "good" intention was against God's will, and Jesus thus judged it to be evil.