The greatest offense is to offend a devotee of the Lord
Srimad-Bhagavatam
SB Canto 3
There are many kinds of offenses, but the greatest offense is to offend a devotee of the Lord.
SB 3.15.35, Translation and Purport: When the doormen of Vaikuṇṭhaloka, who were certainly devotees of the Lord, found that they were going to be cursed by the brāhmaṇas, they at once became very much afraid and fell down at the feet of the brāhmaṇas in great anxiety, for a brāhmaṇa's curse cannot be counteracted by any kind of weapon.
Although, by chance, the doormen committed a mistake by checking the brāhmaṇas from entering the gate of Vaikuṇṭha, they were at once aware of the gravity of the curse. There are many kinds of offenses, but the greatest offense is to offend a devotee of the Lord. Because the doormen were also devotees of the Lord, they were able to understand their mistake and were terrified when the four Kumāras were ready to curse them.