Shirk is the dangerous sin in islam.

 Shirk is the dangerous sin in islam. 

Alhamdulillah, the first and foremost sin in islam is shirk. 

Shirk means to associate partners with Allah. 

Allah says from the Quran, 

An-Nisa' 4:48

إِنَّ ٱللَّهَ لَا يَغْفِرُ أَن يُشْرَكَ بِهِۦ وَيَغْفِرُ مَا دُونَ ذَٰلِكَ لِمَن يَشَآءُۚ وَمَن يُشْرِكْ بِٱللَّهِ فَقَدِ ٱفْتَرَىٰٓ إِثْمًا عَظِيمًا 

Indeed, Allāh does not forgive association with Him, but He forgives what is less than that for whom He wills. And he who associates others with Allāh has certainly fabricated a tremendous sin.

Allah says again, 

An-Nisa' 4:116

Indeed, Allāh does not forgive association with Him, but He forgives what is less than that for whom He wills. And he who associates others with Allāh has certainly gone far astray.

No matter the sin one commits and comes to Allah without shirk, Allah will forgive him.

As reported in hadith,

Abd Allaah (bin Masud) said “I asked Apostle of Allaah(ﷺ) which sin is the gravest?” He replied “That you associate someone with Allaah, while He hascreated you”. I again asked “Which then?” He replied “That you commit adultery with the wife of your neighbor.” Allaah then revealed the following Qur’anic verse in support of the statement of the Prophet (ﷺ) “Those who invoke not with Allaah any other god nor slay such life as Allaah has made sacred except for just cause nor commit fornication.”

Sunan Abi Dawud 2310.

I heard Abu Dharr narrating it from the Apostle (ﷺ) that he observed:

Gabriel came to me and gave me the tidings: Verily he who died amongst your Ummah without associating anything with Allah would enter Paradise. I (the narrator) said: Evenif he committed adultery and theft. He (the Holy Prophet) said: (Yes), even if he committed adultery and theft.

Sahih Muslim 94 a.