Some means of calling people to Allah and renewing faith in their hearts
Question

How can we instill faith in hearts that have grown hard? How can we awaken them to the glory of Allah and make them love Islam and want to be close to Allah? I want to know various convincing, beneficial means that will help to do that – may Allah bless you – through which the seeker of knowledge or the one who specializes in this kind of calling people to Allah can develop this approach and become able to call them in an effective manner and achieve positive outcomes in calling them to righteousness and religious commitment. Then after that, they will automatically offer the person something, and thus foot-dragging and drifting away will stop, and people will become more interested in adhering to the path of Islam. Then at last the awareness of Allah’s greatness, which is sleeping or almost absent in most people’s hearts, will awaken and they will truly realize the extent of the blessings that they are enjoying. Then they will return the favour when they serve this great religion and do their utmost to serve the people of this religion and others. We hope that you will give a complete and satisfactory answer.
Praise be to Allah.People’s hearts are vessels, and the best of them are those which are filled with goodness and guidance, whilst the worst of them are those that are filled with evil and mischief.
One of the early generations said: The hearts of the righteous overflow with goodness whilst the hearts of the evildoers overflow with evil.
End quote from Miftaah Daar as-Sa‘aadah (1/124).
In between these two types of hearts is the heart that has a mixture of both good and evil, and is wavering; sometimes it turns to its Lord, and sometimes it turns to its whims and desires.
So it may be overwhelmed with love of obedience and heeding the commands of Allah and His Messenger, so it remains steadfast for a while, then it becomes overwhelmed by its whims and desires, so it breaks the covenant, disobeys its Lord, obeys its enemy and follows a path other than the path of guidance.
Undoubtedly it is more appropriate for the Muslim to do the right thing in any situation. Therefore if he adheres to righteousness, reward and blessings are more appropriate for him, but if he goes astray, then it is more appropriate for him to repent and turn back to Allah.
When he becomes negligent, the Muslim needs reminders, exhortation, encouragement and warnings, for his heart is still beating with life, even if he drifts away from the company of the righteous and even if he stays away from gatherings of exhortation and reminders.
There follows some beneficial advice for calling heedless Muslims:

Having a good attitude and good manners when calling them; being patient with any offensive responses or negative reactions; responding with smiles and cheerfulness, shaking people’s hands and expressing joy at meeting them.
Opening the door of hope and reminding them of Allah’s rewards; telling them of the vast mercy of Allah and that He loves for His righteous slaves to constantly obey Him, just as He loves for His disobedient slaves to come back to Him, repent and turn to Him, for He rejoices at that.
Seeking to soften hard hearts and making them love to remember Allah by telling them – first of all – about Allah, may He be exalted, and His beautiful names and sublime attributes, for no one learns about his Lord and His names and attributes but he will love Him, and he will love to obey Him and hate to be far away from Him, and he will hate whatever takes him far away from Him.

Then – secondly – he should tell them about what Allah bestows upon those who remember Him of protection, reward and blessings, in this world and the Hereafter.

Encouraging them to read and memorise the Qur’an and to pay attention to reflecting upon it and contemplating it, for that is the basis of guidance to the straight path of Allah, and by means of it the Lord will protect His slave from the tricks of the shayaateen (devils).
Calling them to reflect upon the signs of Allah in the universe, and in their own selves, and the blessings which, if Allah so willed, He could take away from them.
Explaining that what is required in response to the blessings that Allah has bestowed upon them is to give thanks for them. As for being ungrateful for them, not giving thanks for them and using them to disobey Allah, that will lead to loss of those blessings.
Calling them to guard their minds against passing thoughts and whispers of the Shaytaan.

Ibn al-Qayyim (may Allah have mercy on him) said:
Ward of passing thoughts, for if you do not do that, such a thought will become an idea. Ward off the idea, for if you do not do that, it will become a desire. Fight it, for if you do not do that, it will become resolve and determination, and if you do not ward it off it will become a deed, and if you do not counter it with its opposite, it will become a habit, whereupon it will be very difficult for you to rid yourself of it..
End quote from al-Fawaa’id (p. 31).

Teaching them to love obedience and hate disobedience, for obedience is an act of worship of the Most Gracious, and sin is an act of obedience to the Shaytaan. Whoever takes his enemy as a friend and takes his friend as an enemy has committed the most grievous error.
Teaching them about matters of religion, such as the manners and attitudes that Islam encourages and that have to do with developing good attitudes, being a good neighbour, being grateful for blessings, and helping others. Encouraging them to get to know Islam by teaching them these attitudes and manners, and teaching them how the character and attitude of the Prophet (blessings and peace of Allah be upon him) was the Qur’an, and that Allah sent him to perfect good manners and good attitudes.
Pointing out to them how bad it is to stay away from the Knower of the unseen, and that keeping away from Him exposes one to alienation and doom, whereas being close to Him leads to a sense of assurance and strength.
Encouraging them by quoting texts that speak of reward, and alarming them by quoting texts that speak of punishment, for each person is between reward and punishment – if he obeys Allah he will be rewarded, and if he disobeys Him he will be punished, unless Allah pardons him.
Encouraging them to keep company with good and righteous people, and warning them against keeping company with evil and disobedient people, for a man will follow the religion of his close friend.
Putting their hearts at ease by adopting a gentle and easy approach when exhorting them, then shocking them with stern warnings, for each person is between fear and hope.
Always raising their hopes of forgiveness, and avoiding any talk of despair and giving up; explaining to them that there is no room for despair in our religion, for the Lord holds out His hand by night to accept the repentance of those who sinned by day, and He holds out His hand by day to accept the repentance of those who sinned by night.

All of that boils down to the following:
Trying hard to make people feel at ease with the word of the Lord of the worlds and find joy in obeying Him, and to make them feel at ease with the words of the Messenger of Allah (blessings and peace of Allah be upon him) and his Sunnah. The daa‘iyah (caller) should strive to teach them the Book of the Lord of the Worlds and help them to study it, recite it, reflect upon it and learn its teachings as much as possible, by using simple and trustworthy commentaries such as: Aysar at-Tafaaseer by Shaykh Abu Bakr al-Jazaa’iri, at-Tafseer al-Muyassar, or Tafseer as-Sa‘di [now available in English from IIPH (International Islamic Publishing House), Riyadh].
He should also pay attention to teaching the Sunnah of the Prophet (blessings and peace of Allah be upon him), especially the practical aspect thereof. He can take as a basis for such study Riyaadh as-Saaliheen by Imam an-Nawawi (may Allah have mercy on him).
For more information, please see the following books:
Weakness of Faith by Shaykh Muhammad Saalih al-Munajjid
Means of Steadfastness: Standing Firm in Islam by Shaykh Muhammad Saalih al-Munajjid
And Allah knows best.