Three things that let a person taste it
The third one is the hardest to fake.
ثَلاَثٌ مَنْ كُنَّ فِيهِ وَجَدَ حَلاَوَةَ الإِيمَانِ أَنْ يَكُونَ اللَّهُ وَرَسُولُهُ أَحَبَّ إِلَيْهِ مِمَّا سِوَاهُمَا، وَأَنْ يُحِبَّ الْمَرْءَ لاَ يُحِبُّهُ إِلاَّ لِلَّهِ
Thalāthun man kunna fīhi wajada ḥalāwata-l-īmān: an yakūna-Llāhu wa rasūluhu aḥabba ilayhi mimmā siwāhumā, wa an yuḥibba-l-marʾa lā yuḥibbuhu illā lillāh
“Three things: whoever has them finds the sweetness of faith. That Allah and His Messenger are more beloved to him than anything besides them. And that he loves a person for no reason except Allah.”
Where this comes from
The third is that he hates to go back to disbelief the way he would hate to be thrown into a fire. Sweetness is a taste, not a conclusion, which is why the three named are all about what a person wants rather than what he believes.
The full narration
Narrated Anas: The Prophet (ﷺ) said, "Whoever possesses the following three qualities will have the sweetness (delight) of faith: 1. The one to whom Allah and His Apostle become dearer than anything else. 2. Who loves a person and he loves him only for Allah's sake. 3. Who hates to revert to Atheism (disbelief) as he hates to be thrown into the fire
Source
Ṣaḥīḥ al-Bukhārī · 16
- Agreed upon as authentic (al-Bukhārī / Muslim)