Why does God allow His children to sometimes fall into grievous sin? The answer that the Westminster Confession gives us is God uses this to humble them. Here it is below:
The most wise, righteous, and gracious God does oftentimes leave, for a season, His own children to manifold temptations, and the corruption of their own hearts, to chastise them for their former sins, or to discover unto them the hidden strength of corruption and deceitfulness of their hearts, that they may be humbled; and, to raise them to a more close and constant dependence for their support upon Himself, and to make them more watchful against all future occasions of sin, and for sundry other just and holy ends.
Here we see that death and resurrection theme. Sometimes a proud heart can only be humbled when it is brought low by its own sin. God uses this to bring about a spiritual renewal and awakening, resulting in a much closer walk with God.
For those watching, it can be heart breaking. But that is why faith is needed. And if you have a loved one who has been caught in a grievous sin, pray productively that God will use the fall as a means of destroying pride (that’s the death part) and that He will create a new heart that is humble and faithful (that’s the resurrection part).
It is easy to get confused and disheartened when someone close to you falls. But we must keep our eye on the story line and remember that God is faithful and will bring all things to an end that is consistent with His glory and our good. And, someone else’s fall into sin can and should be a sermon to everyone else, a reminder of our own frailty and weakness, and a motivation to take heed to our own souls.