Will God Really Protect Me?

The LORD will protect your coming and going both now and forever.
--- Psalm 121:8

The way I understand protection is the process of keeping away physical harm or sickness, as well as fear and emotional threats.  The psalm ensures protection in both "coming and going."  The threats on our well-being is more likely when we put ourselves out there, move out of our comfort zone.  But I look around me, and I struggle sometimes to see that protection for myself and others.

I'm sure the psalmist means protection for those who trust in the Lord, but I know devout Christians whose lives have been upended by disease or tragedy, agitation or violence, sorrow or pain.  Was their trust weak?  I always want to apply a formula for Bible promises, but it leaves me wondering if my formula is flawed when the anticipated results aren't realized.