According to some Christians, the field of psychology can intrudes too far into the metaphysics of mind and thereby infringe upon Christian theology pertaining to the soul. And so you'll see some Christians take offense at psychology as a 'devil's tool.'
On the other hand, it's not that hard to find Christians who actually work in the field of psychology as either theorists or counselors and who aren't threatened by the various theories found therein.
Personally, I take a middle-ground approach to psychology, and I do so for various reasons, and by middle ground I mean that I'm not much on the earlier theorists like Freud or Jung and I pretty much ignore them. However, I still very much take seriously the biological side of psychology that studies not just behavior but the components of the physical brain, how it functions, and what happens when it malfunctions. This last part is relevant to me because my own mother had schizophrenia and it forced me to learn 'why' she had the perceptual and other cognitive problems that she did.
Peace