The Bible uses the analogy of good and bad fruit to distinguish between what believers and nonbelievers will produce in their lives (Matthew 7:17-20). Genuine followers of Christ will produce good fruit (John 15:8). Jesus says if we abide in Him, we will produce fruit that is growing and desirable (John 15:4-5), as we cannot produce any other kind.
But for those not in Him, Jesus likens them to bad trees that can only produce bad fruit (Matthew 7:18). A bad tree doesn’t produce good fruit every now and again; it produces bad fruit every time. Sure, it might look appealing on the outside, but inside it is rotten and sour. If we consume enough of it, what do you imagine will become of our spiritual health?