Jesus said it, I believe it, that settles it. However, the Holy Spirit inspired Luke (in the Book of Acts), Paul (in the Book of 1 Corinthians and second letter to Timothy), and Peter (in his second letter) to encourage others to - "Be on your guard" also.
I encourage the Christian community and myself to take our spiritual awareness to the next level. At least pay some attention to developing this part of our spiritual walk. Some like the word discerning, some like the word distinguishing, but this is how 1 John 4:1-3 (NIV) says it - "Dear friends, do not believe every spirit, but test the spirits to see whether they are from God, because many false prophets have gone out into the world. This is how you can recognize the Spirit of God: Every spirit that acknowledges that Jesus Christ has come in the flesh is from God, but every spirit that does not acknowledge Jesus is not from God."
I challenge us not only to be on our guard for those that do not acknowledge Jesus is from God, but also challenge us to be on our guard from blatant and subtle attacks from the enemy. Sometimes our attention level can drop to the point where an obvious demonic attack surprises us. Other times we can get so complacent and/or comfortable that Christians don't even know they have been subtly attacked until it is too late.
I do want to offer a warning here. I realize that not every problem and issue we face is of the enemy. I am of the idea that most of the challenges I face come from me and my environment. So, I am encouraging myself as well as other Christians to pause before labeling a situation "of the enemy". However, if you can discern through prayer, study of The Word, and trusted consultation that you are under demonic attack or there is potential for an attack, then "Be on your guard".
Jesus told His disciples the condition He was sending them out into - "like sheep among wolves". Nothing like jumping right into the deep-end. But Jesus did not stop there (thank you Lord). He threw in a "Therefore". And anytime you see a "Therefore", you have got to ask what it is there for. The "Therefore" says - Because of this condition ("sending you out like sheep among wolves") I have special instructions.
My paraphrase of the next part is be cunning and strategic as a sneaky snake, but keep integrity (innocence as a dove) intact.
Notice that the charge to "Be on your guard" that comes afterward is "against men" not against the Devil. Again, we can fall from our own mistakes, from an attack from the enemy of our souls, or like Jesus says here - from our own human brothers/sisters. The keys are to "test the spirits", have a clever strategy, and keep your integrity. But before all this, always - "Be on your guard"!