Praise be to Allah, and peace and blessings be upon the Messenger of Allah. To proceed:
True faith is not proven merely by claiming it. Rather, its effects become evident during trials and temptations. Allah, the Exalted, says:
"Do people think that they will be left to say, 'We believe,' without being tested?"
(Qur'an 29:2)
Whoever remains steadfast in the face of doubts, strives against personal desires, and holds firmly to the truth, demonstrates the soundness of their faith. However, a person who is shaken by every doubt or allows desires to lead them into abandoning acts of worship and committing sins without repentance shows weakness in faith and should return to Allah with sincere repentance.
Among the signs of true faith are:
The Prophet ﷺ said:
"Whoever is pleased by his good deeds and saddened by his bad deeds is a believer."
As for the whispers (waswasah) that Satan casts into the heart, they do not harm the believer as long as they dislike them, reject them, ignore them, seek refuge in Allah, and do not follow them, just as the Prophet ﷺ instructed.
Conclusion
The soundness of faith is shown by steadfastness in obedience, adherence to the truth, hatred of sin, and eagerness to repent after falling into error. Mere satanic whispers do not harm the believer if they reject them and do not act upon them. Their remedy is to ignore them, seek refuge in Allah, increase in acts of worship, and keep the company of righteous believers.
And Allah knows best.