Satan could speak to Jesus, but could not force him to do anything (Luke 4:1-13). Satan might attack a person by harassing him, but a Christian who knows his authority in Christ can make him leave, just as Jesus made him leave. The Bible says that the devil is like a roaring lion, seeking whom he may devour (1 Peter 5:8). So he can attack. But the Bible also says, “Resist the devil, and he will flee from you” (James 4:7). So we can win.
Based on the little you have told me, I should not offer a definite opinion on whether your friend is under demonic attack, and whether the sudden and strange physical condition is directly related to the occult sin he committed. If it is a demonic attack, then it seems that his depression was the beginning of it, and the things that followed were subsequent stages that made the situation worse. King Saul had been harassed by an evil spirit for a time, but when he became desperate, he visited someone with a familiar spirit, a witch or necromancer, and it made everything worse for him.
Your friend’s spiritual problem started earlier, and the fact that he pursued or permitted an illegitimate attempt to alleviate it indicates a deeper issue with him than simple depression or a physical condition. This has to be addressed, otherwise he would be targeting the symptoms only, and something worse might happen later (John 5:14, Luke 11:26). Learning, believing, and affirming the word of God must be the major part of the solution. He has to unravel wrong beliefs and attitudes, and change his behavior. This is more important than addressing the symptoms.
Some sicknesses are more likely or directly caused by demons than others. A physical illness without an apparent physical defect is more probably a demonic attack. For example, if a person has a broken eardrum, or if he was born without one, then it is probably not demonic, but it still might be. We would pray for the eardrum to be healed or command an eardrum to be created. If a person is deaf but there seems to be nothing wrong with him even under doctor’s examination, then it could be a demonic attack. Sometimes a minister either cannot investigate or does not need to. He would know one way or another by the Spirit, and command the spirit of deafness to leave.
This kind of attack can happen to a Christian, especially under certain conditions, such as accident, trauma (intense and sudden fear, grief, etc.), abuse (physical, verbal, sexual, etc.), false teaching, lack of faith, grievous sin, among others. Although we can discuss the matter, we often cannot arrive at definite conclusions about specific cases. However, we know the essential piece of information. We know that there is healing in Jesus Christ. A Christian can immediately return to fellowship with God through repentance and teaching, affirming the goodness of God and the atonement of Christ, and demand the attack to cease. This is ultimately the best solution for all kinds of conditions, because if someone else does this for him, there is the chance that the attack will return. So even if someone else does it, the person must still follow up with repentance, teaching, and so on.
As for the method for casting out demons by a minister, it will somewhat vary according to the person’s knowledge, disposition, spiritual endowment, and degree of faith. For the sake of brevity, we will consider treatment for Christians and non-Christians at the same time, even though Satan cannot affect Christians to the same extent. And keep in mind that the vast majority of those who claim to be Christians are in fact non-Christians. Fake Christians, including pastors and theologians, can be thoroughly demon-possessed. Perhaps this is why they resist the Spirit of God. Cessationism is Satan’s ultimate protection.
Those who are very invested in the “deliverance” ministry often take the approach of assuming the condition (that everyone has demons), probing the mind (digging into the childhood like a psychologist), guiding the person (through repentance, forgiving others, renouncing the occult, inherited curses, etc.), forcing the manifestation (making the demons take over the person to a degree), addressing the demons (asking for their names, etc.), and then casting them out. This is not the biblical model. Most of these steps are overemphasized, and often unnecessary.
Sometimes there is mention of a thing or two about the person’s childhood. Sometimes it is necessary to renounce the occult. However, these steps often seem necessary only because they are assumed to be necessary. The spirits can often be cast out immediately, and then these things can be taken care of as part of the person’s continuing development in the Christian faith. Certainly, if a person’s mind has been taken over because of involvement in the occult, such that he cannot offer his consent, the spirits can be cast out first, and then he can be told to renounce his former life. If he refuses, then as Jesus said, the evil spirits will likely return to him along with ones that are even worse.
Perhaps it seems anticlimactic to cast out an army of demons in ten seconds, but this is what we should often expect. Don’t cast out demons to cause a spectacle or to show off your talent. Do it to demonstrate the kingdom of God: “But if I drive out demons by the Spirit of God, then the kingdom of God has come upon you” (Matthew 12:28). If you are such a man of faith and power, you would do it quickly. You would tell the spirits to shut up and get out. David did not give Saul a questionnaire. He just played music and the evil spirit would depart (1 Samuel 16:23). Music was one way the Spirit of God worked through him. It succeeded, and it was marvelous at the time, but it actually demonstrated weaker authority than what an ordinary Christian possesses (Luke 7:28). I don’t need to play music. I don’t need a harp. Sometimes I only need to show up (Acts 19:15). This was probably why the people were astonished at Jesus’ approach. It was possibly unprecedented — without tools, without rituals or gimmicks, and without praying for God to act, he spoke to the spirits and they obeyed him (Mark 1:27). We wield the same authority in the name of Jesus, and take the same approach when we cast out demons (Acts 16:18).
A Christian who speaks with spiritual authority might force some spirits into manifestation as he exercises the ministry of preaching and teaching. God has ordained and anointed him, and divine power is evident as the man speaks. The spirits might scream out, and cause their victims to roll on the floor, foam at the mouth, or speak in strange voices. They did this under the ministry of Jesus. When this happens, the Christian should command the spirits to shut up and leave. Or, the spirits would sometimes leave silently while such a man speaks, without causing any trouble. The victims would later discover that they have departed, and that the associated symptoms have ceased. Some ministers might not speak with such power that the spirits would be compelled to reveal themselves, but when the spirits take over their victims in other situations, probably under the normal course of their lives, then they can be cast out.
If there is reason to suspect that a person has evil spirits even though there has been no manifestations, they can also be addressed and commanded to leave. There is this practice in some ministries where the minister commands the demons to show themselves even when the victims are not exhibiting signs of demon control at the moment. I am against this. The biblical approach is to respond to these manifestations when they happen “naturally,” if they need to happen at all. They would occur under the pressure of a speaking ministry, or the victims would be brought to the minister while the demons have control over them.
Moreover, do not consult the demons themselves on how to cast them out. They are liars. Speak to them as little as possible. You might assume it should be unnecessary to make a point of this, but some people have an unhealthy fascination with demons, and with their power over demons. However, if they are truly spiritual, they would be more impressed that their names are written in heaven than with the fact that evil spirits must submit to them in the name of Jesus (Luke 10:17-20). Authority over demons in the name of Jesus should be taken for granted, but we can never thank God too much for the gift of Jesus Christ.
A Christian who ministers with authority can often force demons to leave without directly addressing them. Using the example of a deaf person again, if I do not find any reason to bring up a spirit, I can just command the ears to be opened, and the ears could open even if the problem is not physical but demonic. This is because the intention in my command is for the ears to open. I don’t really care why they were closed in the first place. I don’t need to know how it is to be done. If there is enough power behind it, the demons could be forced out anyway.
Avoid most ministries that specialize in “deliverance ministry,” because they are too obsessed with demons. Their first method is to assume the presence of demons, probe the person, and cast out the spirits. When someone is brought to me, unless the demons are already in manifestation — moving and speaking through the person, and controlling him — I would take the teaching route first, because the person would need the teaching even if you first cast out the demons. When a Christian is strong in his spirit and exercises an authoritative ministry of preaching and teaching, some people are automatically delivered as he speaks. The diseases and demons are forced out, and the people also grow stronger as they listen, which will eventually enable them to fend for themselves.
Deliverance from evil spirits comes under the healing ministry of Christ and his work of atonement (Matthew 8:16-17). The Christian has the right to be free. He can declare, “Jesus took my infirmities and bore my sicknesses. I will not tolerate this. I command the demonic attacks to cease, and I command my body to receive healing and return to normal.” He might need to say it one time, or insist on it a thousand times, but he will succeed if he has faith. Jesus resisted three times before Satan left him (Luke 4:1-13). When a Christian receives deliverance this way, that thing is less likely to return. And if it tries, he will know how to overcome it again.
As for Matthew 12:29, it is more about Jesus’ ministry strategy than about casting out demons from individuals. It includes all the things he did — preaching the gospel, healing the sick, casting out demons, and ultimately, saving his people. He came to destroy the works of the devil (John 10:10, 1 John 3:8), not just by casting him out of individuals (Acts 10:38), but by all that he did. He broke the devil. And then he plundered the devil, and continued to plunder him through his disciples. In context, the verse means that Satan would not do this to himself, but Jesus was doing that to Satan.