Double-minded

Double-minded, duplicitous, two-faced - the list could go on.
All imply untrustworthiness. And the opposite -

single-minded, simple, single-eyed (see Matt.6:22, AV)
- imply sincerity, honesty, transparency, virtues that Christians are bound to aspire to.

Ask in faith, never doubting, for the one who doubts is like a wave of the sea, driven and tossed by the wind; for the doubter, being double-minded and unstable in every way, must not expect to receive anything from the Lord. James 1:6-8