How can faith be restored after it has left the heart and worldly matters have become the greatest concerns?

Faith is not entirely removed from a Muslim's heart except by committing one of its nullifiers. The creed of Ahl al-Sunnah (definition) wa al-Jama'ah is that they do not declare a person who commits sins, even major ones, to be a disbeliever, as long as they do not deem them permissible. However, faith decreases with sins and transgressions, and increases with acts of obedience. Therefore, when a servant commits sins, their faith decreases, but absolute faith is not removed from them. If they repent, their faith returns and increases. It is obligatory for a Muslim to repent from sins, increase good deeds, associate with righteous people, and seek beneficial knowledge.