Fire and brimstone, and the atonement
A leaky faucet is usually the result of a hole or crack in the washer that stops the flow of water. If we remove the offending washer and replace it with a new one, we can stop the flow. If we ignore the leak and fail to replace the washer, we suffer the consequences when we get the water bill at the end of the month. Ignored for months, and the price becomes significant.
To answer your question, Auvie, of when does the atonement take place, the atonement already happened, in the garden of Gethsemane, on the cross at Calvary, and in the garden tomb. The effects of the atonement take place every day in the lives of those who daily repent and strive to be and do better. And the sole purpose of the atonement of Jesus Christ is to prevent the need for “fire and brimstone” in our lives.
We know from the laws of physics that for every action there is an equal and opposite reaction. We also know from the laws of God that for every action there is a consequence. If we sin, we feel remorse and pain of soul for having transgressed the laws of God. The greater the sin, the stronger the feelings of guilt. If we heed this pain, it leads us to repentance, and when we have fully repented we are forgiven, the effects of the atonement have taken place in our life, and we are once again entitled to the presence of the Holy Spirit. If we do not repent, the Spirit cannot reside with us, and eventually we become numb to the remorse and pain we might otherwise feel. Alma had gone about committing evil against the Church for likely many years, and at some point, he had pushed past the pain to the point he simply was devoid of remorse and conscience.
In the case of Alma, the Younger, he tells us of the pain he suffered for three days and nights, being in the gall of bitterness. At the time the angel appeared to him, all the remorse, the pain, and the bitterness that comes with the sins he committed came flooding back, which is why he was racked with torment. As he said, however, “I am snatched, and my soul is dpained no more.” Interestingly, even though the atonement had not yet taken place, nevertheless Alma felt the effects of it as he “[waded] through much tribulation, repenting nigh unto death.” We are spared that torment when we make repentance a daily process. When we feel remorse, and accept that as an invitation to seek God’s forgiveness when it happens, we are not faced with a flood of guilt. And our repentance and forgiveness is made stronger through weekly participation in the Sacrament, “that we may have His Spirit to be with us.”