Once God has spoken;
Twice I have heard this:
That power belongs to God;
And lovingkindness is Yours, O Lord,
For You recompense a man according to his work.
These two verses go together.
The first thing that the Psalmist has learned from God is that He is powerful. He is sovereignly in control. There is none like Him in the extent of power, insomuch as power belongs to him. All that happens, happens because of Him.
That alone would be an element of fear. To have one being that was in control to do as He wishes, that alone would be a thing of dread.
This power that God holds does not come alone. It is perfectly coupled with His grace (His lovingkindness). Together, these two qualities are something to run to, not away from. This culminates the idea of God as a stronghold and a fortress. There is safety in Him.
The one with all power is seeking out our good in all things.
This is a combination worthy of our praise, our adoration, and our loyalty.