Double-mindedness is found in the falsely converted. Both "minds" are actually still dealing in the flesh, however. You say to yourself you are doing things in and for Jesus, but at the same time you are actually being led by other masters, other gods of your life. To serve two masters is double-mindedness. You say you want God--that you believe . . . and you follow the rules and laws of God, so in one sense you have a "mind" that "looks" to scripture and submits to certain Biblical behavior . . . . But inwardly you really are still led by pride and self rule . . . being led by the fallen, old self.
The mind you have which dictates and guides your actions . . . is still of and for the world, though a contrary part of your mind claims God. Religionists are this type--the Pharisees and scribes are the archetype of double-mindedness, who Jesus so vociferously exposed--likewise, those mixing world and temple . . . to the point of putting a whipping on them in a burst of righteous indignation.
"The light of the body is the eye: if therefore thine eye be single, thy whole body shall be full of light."
"No man can serve two masters: for either he will hate the one, and love the other; or else he will hold to the one, and despise the other. Ye cannot serve God and mammon."
I have been mentioning frequently the mystery of "sameness and difference" with respect to, for instance, the dual nature a believer obtains where we are born of spirit AND yet we retain . . . for a time, the persisting entropy of the old self. Also, where Jesus was both man AND God. Similarly, how God is one . . . AND three persons . . . .
Seeing this . . . is not being double-minded, I just wanted to mention, in case someone might mistake.
One of the--if not the most . . . powerful means of staying on track (within the strait and narrow) to keeping a "single eye" and avoided the pitfalls of double-mindedness . . . is to foremost, in all things, be looking at and working for . . . the glory of God. It is so very simply put, yet is one of the most profound truths and realizations, once you see it . . . to: "Whether therefore ye eat, or drink, or whatsoever ye do, do all to the glory of God."
EVERYTHING . . .both reduces AND expands . . . to this single purpose and cause . . . .
In any and every situation . . . as the verse alludes to . . . one can and should consider how the glory of God is being satisfied, addressed, heralded . . . . Not our, but His glory, be done.