The last verse of the previous chapter would show us that this is faith in what God has done in delivering Christ for our offenses and raising Him for our justification.
It is the fruit of simply believing that God has provided in Christ a perfect atonement for sin. The work was done at the cross, and His resurrection is the witness of God's acceptance of the work His own Son has completed.
Oh, the peace forever flowing
From God's thoughts of His own Son,
Oh, the peace of simply knowing
On the cross that all was done.
Peace with God, the blood in heaven
Speaks of pardon now to me:
Peace with God! The Lord is risen!
Righteousness now counts me free.
A mind at "perfect peace" with God:
Oh, what a word is this!
A sinner reconciled through blood:
This, this indeed is peace!
By nature and by practice far,
How very far from God!
Yet now by grace brought nigh to Him,
Through faith in Jesus' blood.
So nigh, so very nigh to God,
I cannot nearer be;
For in the person of His Son,
I am as near as He.
So dear, so very dear to God,
More dear I cannot be;
The love wherewith He loves the Son,
Such is His love to me.
Why should I ever careful be,
Since such a God is mine?
He watches o'er me night and day,
And tells me, "Thou art mine."