Psalm 28

Do not drag me away with the wicked,

with those who do evil,

who speak cordially with their neighbors

but harbor malice in their hearts.

Repay them for their deeds

and for their evil work;

repay them for what their hands have done

and bring back on them what they deserve.

You just can't trust people.

That seems to be the problem here. How often have you known someone who is nice to your face, but trashes you as soon as you walk away? Maybe you've seen someone who claims to be another person's good friend, yet they constantly ridicule them and tear them down when that person is gone.

Or, perhaps, you are the guilty party?

However you know the duplicitous human, you know they can't be trusted. They are unreliable at best and an assaulting enemy at worst. Someone we need protection from.
Now, counter that view of two-faced people with this view of God:

Praise be to the LORD,

for he has heard my cry for mercy.

The LORD is my strength and my shield;

my heart trusts in him, and he helps me.

My heart leaps for joy,

and with my song I praise him.

The LORD is the strength of his people,

a fortress of salvation for his anointed one.

Save your people and bless your inheritance;

be their shepherd and carry them forever.

The people leave you worn out and weakened- God gives you strength, protection and something to trust. In a cynical world where it is probably right to be leery of people, God is stable and reliable. His word is His bond- literally. While you can't trust your neighbor to be honest- or even your family sometimes- God will not leave you hanging, He will never lie.

It is because of this that God is a "Fortress of Salvation." There will be no rug pulled out from under you with God- He will keep His word.

And that alone makes His salvation secure.