Shaykhul Islaam Ahmad ibn Taymeeyah 728AH (May Allah have mercy on him) said,

ولا أنفع للقلب من التوحيد وإخلاص الدين لله ، ولا أضرَّ عليه من الإشراك.

مجموع الفتاوى [٦٥٢/١٠]

“There is nothing more beneficial for the heart than Tawheed and sincerity in the religion for Allah, and there is nothing more harmful for the heart than shirk”

Majmu’ Fataawa 10/652