The answer from God’s word:
Though Jesus did much work among man, He only completed the redemption of all mankind and became man’s sin offering, and did not rid man of all his corrupt disposition. Fully saving man from the influence of Satan not only required Jesus to take on the sins of man as the sin offering, but also required God to do greater work to completely rid man of his disposition, which has been corrupted by Satan. And so, after man was forgiven his sins, God has returned to flesh to lead man into the new age, and begun the work of chastisement and judgment, and this work has brought man into a higher realm. All those who submit under His dominion shall enjoy higher truth and receive greater blessings. They shall truly live in the light, and shall gain the truth, the way, and the life.
… when God becomes flesh this time, His work is to express His disposition, primarily through chastisement and judgment. Using this as the foundation, He brings more truth to man, shows more ways of practice, and so achieves His objective of conquering man and saving man from his corrupt disposition. This is what lies behind the work of God in the Age of Kingdom.
from Preface to The Word Appears in the Flesh
The work in the last days is to speak words. Great changes can be effected in man through the words. The changes now effected in these people on acceptance of these words are much greater than that of people in the Age of Grace on acceptance of those signs and wonders. For, in the Age of Grace, the demons went away from man with the laying on of hands and prayer, but the corrupt dispositions within man still remained. Man was healed of his sickness and forgiven his sins, but the work for just how the corrupt satanic dispositions within man could be cast away was not done in him. Man was only saved and forgiven his sins for his faith, but the sinful nature of man was not taken away and still remained within him. The sins of man were forgiven through God incarnate, but it does not mean that man has no sin within him. The sins of man could be forgiven through the sin offering, but man has been unable to resolve the issue of just how he can no longer sin and how his sinful nature can be cast away completely and be transformed. The sins of man were forgiven because of the work of God’s crucifixion, but man continued to live in the old, corrupt satanic disposition. As such, man must be completely saved from the corrupt satanic disposition so that the sinful nature of man is completely cast away and never again develops, thus allowing the disposition of man to be changed. This requires man to understand the path of growth in life, the way of life, and the way to change his disposition. It also needs man to act in accordance with this path so that the disposition of man can gradually be changed and he can live under the shining of the light, and that he can do all things in accord with the will of God, cast away the corrupt satanic disposition, and break free from Satan’s influence of darkness, thereby emerging fully from sin. Only then will man receive complete salvation. When Jesus was doing His work, man’s knowledge of Him was still vague and unclear. Man always believed that He was the son of David and proclaimed Him to be a great prophet and the benevolent Lord who redeemed man’s sins. Some, based on faith, became healed just by touching the edge of His garment; the blind could see and even the dead could be restored to life. However, man could not discover the corrupt satanic disposition deeply rooted within him and neither did man know how to cast it away. Man received much grace, such as the peace and happiness of the flesh, the blessing of the entire family upon the faith of one, and the healing of sicknesses, and so on. The rest were the good deeds of man and their godly appearance; if man could live based on such, he was considered a good believer. Only such believers could enter heaven after death, which means that they were saved. But, in their lifetime, they did not understand at all the way of life. They merely committed sins, then made confession in a constant cycle without any path toward a changed disposition; such was the condition of man in the Age of Grace. Has man received complete salvation? No! Therefore, after that stage was complete, there is still the work of judgment and chastisement. This stage makes man pure through the word so as to give man a path to follow. This stage would not be fruitful or meaningful if it continued with the casting out of demons, for the sinful nature of man would not be cast away and man would only stop upon the forgiveness of sins. Through the sin offering, man has been forgiven his sins, for the work of the crucifixion has already come to an end and God has prevailed over Satan. But the corrupt disposition of man still remains within them and man can still sin and resist God; God has not gained mankind. That is why in this stage of work God uses the word to reveal the corrupt disposition of man and asks man to practice in accordance with the right path. This stage is more meaningful than the previous one and more fruitful as well, for now it is the word that directly supplies man’s life and enables the disposition of man to be completely renewed; it is a stage of work more thorough. Therefore, the incarnation in the last days has completed the significance of God’s incarnation and completely finished God’s management plan for the salvation of man.
… Therefore, man cannot be fully saved from his sins by the healing of sickness and casting out of demons and cannot be fully made complete by the manifestation of signs and wonders. The authority to heal and cast out demons only gives man grace, but the flesh of man still belongs to Satan and the corrupt satanic disposition still remains within man. In other words, that which has not been made clean still belongs to sin and filth. Only after man has been made clean through words can he be gained by God and become sanctified. If no more is done than casting out the demons within man and redeeming him, that is only seizing him from the hands of Satan and returning him to God. However, he has not been made clean or changed by God, and he remains corrupt. Within man still exists filth, opposition and rebelliousness; man has only returned to God through redemption, but man has no knowledge of Him and still resists and betrays God. Before man was redeemed, many of Satan’s poisons were already planted within him. After thousands of years of Satan’s corruption, man already has within him a nature that resists God. Therefore, when man has been redeemed, it is nothing more than redemption, where man is bought at a high price, but the poisonous nature within has not been eliminated. Man that is so defiled must undergo a change before being worthy to serve God. Through this work of judgment and chastisement, man will fully come to know the filthy and corrupt substance within him, and he will be able to completely change and become clean. Only in this way can man be worthy to return before the throne of God. All the work done this day is so that man can be made clean and be changed; through judgment and chastisement by the word, as well as refinement, man can cast away his corruption and be made pure. Rather than deeming this stage of work to be that of salvation, it would be more apt to say it is the work of purification. In truth, this stage is that of conquest as well as the second stage of salvation. Man is gained by God through judgment and chastisement by the word; through the use of the word to refine, judge and disclose, all of the impurities, notions, motives, and individual hopes within man’s heart are completely revealed. Though man has been redeemed and forgiven of his sins, it is only considered as God not remembering the transgressions of man and not treating man in accordance with man’s transgressions. However, when man lives in the flesh and he has not been set free from sin, he can only continue to sin, endlessly revealing the corrupt satanic disposition. This is the life that man leads, an endless cycle of sin and forgiveness. The majority of men sin in the day only to confess in the evening. As such, even if the sin offering is forever effective for man, it would not be able to save man from sin. Only half the work of salvation has been completed, for man still has corrupt disposition. … it runs deeper than sin, planted by Satan and deeply rooted within man. It is not easy for man to become aware of his sins; man is unable to recognize his own deeply rooted nature. Only through judgment by the word can such effects be achieved. Only thus can man gradually be changed from that point onward.
from “The Mystery of the Incarnation (4)” in The Word Appears in the Flesh