The fining pot is for silver, and the furnace for gold: but the LORD trieth the hearts. Proverbs 17:3

The Lord might easily have taken every one of us home to heaven the moment we were converted. Certainly, His omnipotence was equal to the task of our immediate perfect sanctification. If the dying thief was rendered fit to be in Paradise the same day on which he believed, so might each one of us have been made ready to enter heaven; but it has not so pleased God. We doubt not that there are myriads before the eternal throne who have reached the abode of bliss without treading the winepress of affliction.

Trial in the Christian church is the Lord’s fining pot, which is never off the fire. It has this excellent effect, that it separates the precious from the vile. As long as the church exists, I suppose she will have traitors amongst her number, for if Judas intruded under the watchful eye of the Chief Shepherd, we may be pretty sure that many a Judas will elude the far less watchful eyes of the minor shepherds. Because trial and persecution test men’s professions, they are used as the winnowing fan in the Lord’s hand, as it is written, “He will thoroughly purge His floor.” In persecution, the mere professors, the camp-followers and hangers-on, soon flee away, for they have no heart to true religion when the profession of it involves the cross. They could walk with Jesus in silver slippers, but they cannot travel with Him when His bleeding feet go barefoot over the world’s rough ways, and therefore they depart every man to his own, and we may say of them “They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us: but they went out, that they might be made manifest that they were not all of us.” So that trial as a permanent institution is of much service to the church in promoting her purity, and we are bound to praise the Lord whose fire is in Zion and His furnace in Jerusalem. ~ C.H. Spurgeon

https://www.blueletterbible.org/Comm/spurgeon_charles/sermons/1277.cfm