For who is God save the LORD? or who is a rock save our God?
Psalm 18:31 KJV
We continue our look at the Psalms and see how they point to or testify of Jesus as he indicated in John 5 and Luke 24 when he said:
Search the scriptures; for in them ye think ye have eternal life: and they are they which testify of me.
John 5:39 KJV
And he said unto them, These are the words which I spake unto you, while I was yet with you, that all things must be fulfilled, which were written in the law of Moses, and in the prophets, and in the psalms, concerning me.
Luke 24:44 KJV
This article, and others found in Questions From the Psalms, focus specifically on questions taken directly from the book of Psalms and illustrates how they speak of Jesus.
The question asked here begins with For, because it is a continuing thought from the previous verse, which ended with this ….. he is a buckler to all those that trust in him.
As for God, his way is perfect: the word of the LORD is tried: he is a buckler to all those that trust in him.
Psalm 18:30 KJV
Therefore, the psalmist was saying, we can confidently live by the word of the LORD, for when we trust his word i.e. when we trust him, he (the word of the LORD) will be a shield (buckler) to protect us. And here we are able to see how this concerns/testifies Jesus.
Therefore whosoever heareth these sayings of mine, and doeth them, I will liken him unto a wise man, which built his house upon a rock: [25] And the rain descended, and the floods came, and the winds blew, and beat upon that house; and it fell not: for it was founded upon a rock.
Matthew 7:24-25 KJV
The word of the LORD is Jesus, so to trust in the word is to trust Jesus. Those who trust him build their lives upon him (his word is the foundation of their life), and in doing so are shielded from the enemy when he comes in like a flood.
To the person who has the LORD as their shield and their rock, the LORD who is God communicates, that they can rest easy knowing that the enemy cannot mount a successful attack against them no matter the frequency, strength or mode of his attack.
As a rock he is immovable and thus, we will not be moved with the lies of the enemy if we stand on the rock.
Before Paul was converted, he sought to kill all who followed Jesus, but when he encountered God on the road to Damascus and was asked “why are you persecuting me?” This followed:
And he said, Who art thou, Lord? And the Lord said, I am Jesus whom thou persecutest: it is hard for thee to kick against the pricks.
Acts 9:5 KJV
Only after this encounter did Paul receive the revelation that the GOD of Israel, is none other than Jesus. Moved by the Spirit, he wrote:
When the children of Israel drank water from a rock in the wilderness scripture says this of the rock: And did all drink the same spiritual drink: for they drank of that spiritual Rock that followed them: and that Rock was Christ.
1 Corinthians 10:4 KJV
So the LORD who is God, the rock of our salvation is none other than Jesus the Christ.

To reject Jesus is to reject the rock and without the rock (who is identified to be GOD by the psalmist), our lives (house) will be washed away by the flood of the enemy.
Recommended reading Is Jesus God?