إِلَّا مَنْ أَتَى اللَّهَ بِقَلْبٍ سَلِيمٍ
Except him who brings to Allāh a clean heart [clean from Shirk (polytheism) and Nifāq (hypocrisy)]. Ash-Shu’ara:89
Shaykh Rabī ibn Hādi Al-Madkhalī (hafidahullāh):
Ponder, is your heart sound?
The sound heart is that which fights Shirk and rejects falsehood, it accepts the truth and refutes falsehood with all its might, this is the sound heart. So, if you notice that you accept the truth from whoever brings it to you (you accept the truth; the truth guides you and your goal is the truth) then by Allāh your heart and intellect is sound.
And, if you realise that you reject the truth when it comes to you from a certain direction, and you accept falsehood when it comes from another direction and you behave as if it is the truth; then know that your heart is sick.
Majmu’ Ar- Rā’iq Pgs. 485-486