Is a person's intention to commit a sin followed by immediate repentance considered a mistake, and is this intention considered a sin or a major sin?

If a person intends to commit a sin but does not act upon it or take steps towards it, and does not firmly resolve to do it, then he is not sinful merely for that intention. This is based on the Prophet's (peace be upon him) saying: "And whoever intended to commit a bad deed but did not do it, it will not be written against him." The danger of intending to commit a sin and then repenting lies in the possibility that death may overtake him before repentance, or while he is committing the sin, and deeds are judged by their conclusions. He might also not be granted the opportunity to repent as a punishment. Therefore, a servant must fear Allah and beware of the steps of Satan, who beautifies sin and promises repentance afterwards, as Allah Almighty says: "And do not follow the footsteps of Satan. Indeed, he is to you a clear enemy. He only orders you to evil and immorality and to say about Allah what you do not know."