All praise is due to Allah, and may Allah's peace and blessings be upon His Messenger.
Merely feeling khushu' (humble devotion) or inner peace during an act of worship is not evidence that a religion is true, nor can it be used as proof that a particular belief is correct. The sweetness and tranquility experienced in worship are a result of true faith, not the evidence by which the truth of faith is established. Truth is recognized through authentic revelation and sound reason, not through emotions and personal feelings alone.
Non-Muslims may also experience feelings of peace, devotion, or emotional attachment during their acts of worship. This is because people naturally become attached to what they believe, and Satan may beautify falsehood in their eyes. Allah says:
"And Satan made their deeds appear attractive to them."
(Qur'an 8:48 and similar meanings elsewhere in the Qur'an.)
Therefore, such feelings do not prove the correctness of their beliefs.
As for khushu' in the Islamic sense, it is the humility, reverence, and awe of the heart before Allah. Its effects become visible through calmness, attentiveness, and sincere devotion in worship. This true khushu' is founded upon Tawhid (the Oneness of Allah), purity, and following the guidance of the Prophet Muhammad ﷺ. It produces righteousness in both the heart and one's actions. Allah says:
"Indeed, prayer prevents immorality and wrongdoing."
(Qur'an 29:45)
To strengthen one's awareness of Allah's closeness and experience the sweetness of faith, a Muslim should establish sound belief, adhere to the Sunnah of the Prophet ﷺ, perform both the obligatory and voluntary acts of worship, remain constant in supplication (du'a), and seek beneficial Islamic knowledge. These are among the greatest means of remaining steadfast and increasing in certainty.
And Allah knows best.