Zina leads to Loss Of Iman
A person with full imān would be aware of Allāh's watchfulness over him. He would be afraid to disobey Allāh (سُبْحَانَهُ وَتَعَالَىٰ) especially by committing a kabīrah (major sin) like zinā. Committing a kabirah is an indication of poor iman, at least while one is in the process of committing it.
Abū Hurayrah (رضي الله عنه) reported that Allah's Messenger (ﷺ) said:
_*“A person who commits zinā does not commit it while he is a (full) believer; nor does he drink alcohol while he is a believer; nor does a thief steal while he is a believer.”*_
[Recorded by al-Bukhari, Muslim, and others]
Thus, while committing a major sin, a Muslim is in a state similar to that of disbelief. When he commits zinā, īmān departs from his heart and hovers over his head like a cloud. Only after he ceases committing zinā would īmān come back to him. Abū Hurayrah (رضي الله عنه) reported that Allāh's Messenger (ﷺ) said:
_*“When a person commits zinā, īmān comes out of him until it is like a cloud (over him). When he stops, īman returns to him.”*_
[Recorded by Abu Dawud and al-Hakim. Verified to be authentic by al-Albani (as-Sahihah no. 509)]