The Catechism in Questions & Answers

60. What does it mean to say: “God is the Perfect One”?

God’s works are good and His ways are right. He does not act on the basis of any external necessities or constraints, but solely on the basis of His own perfect will. God is completely free in His decisions.
God’s perfection incorporates truth. With God there is no lie, deception, or uncertainty, nor is there any difference between His will and His actions. Human beings can experience the perfection of God in Jesus Christ, because Jesus Christ was the only one on earth who was perfect, that is, without sin, without error, in word or deed.

“As for God, His way is perfect; the word of the Lord is proven; He is a shield to all who trust in Him.”
Psalm 18: 30