“Again the next day after John stood, and two of his disciples; and looking upon Jesus as He walked, he saith, Behold the Lamb of God.”- John 1:35-36

When John saw Jesus on that memorable day, he, first of all, beheld Him himself and then he said to others, “Behold the Lamb of God.” “Looking upon Jesus as He walked,” steadfastly beholding Him, watching Him, gazing with humble admiration at Him, he said, “behold the Lamb of God!” Brethren, we cannot preach what we have not practiced. If these eyes have never looked to Jesus, how can I bid your eyes look at Him? Beholding Him, I found peace to my soul; I, who was disposed even to despair, rose from the depths of anguish to the heights of joy by looking unto Him; and I therefore dare to say to you, “Behold the Lamb of God!” Oh, that each one of you might believe our testimony concerning Jesus and look to Him and live!

What did John mean by saying, “Behold”? In the Latin, ecce, is a note of admiration, of wonderment, of exclamation. “Behold the Lamb of God!” There was nothing of greater wonder ever seen than that God Himself should provide the Lamb for the burnt offering, that He should provide His only Son out of His very bosom, that He should give the delight of His heart to die for us. Well may we behold this great wonder. Angels admire and marvel at this mystery of godliness, God manifest in the flesh; they have never left off wondering and adoring the grace of God that gave Jesus to be the Sacrifice for guilty men. Behold and wonder, never leave off wondering; tell it as a wonder, think of it as a wonder, think of it as a wonder, sing of it as a wonder at this glorious Lamb of God. ~ C.H. Spurgeon

https://www.blueletterbible.org/Comm/spurgeon_charles/sermons/2329.cfm