Praise be to Allah, and peace and blessings be upon the Messenger of Allah.
These thoughts are among Satan's whispers, especially when they come after a Muslim has increased in worship and become more devoted to their religion. When Satan fails to prevent a servant from worship, he tries to lead them into doubts and misconceptions.
The Prophet ﷺ informed us that Satan casts such whispers into the hearts of the believers, and he instructed us to ignore them and not dwell on them.
The answer is no.
Inner peace is not the standard by which truth is recognized. A person may feel comfortable with what they have grown accustomed to or were raised upon, even if it is false.
The true criterion is what is found in the Book of Allah, the Sunnah of His Messenger ﷺ, and the understanding of the Companions and the righteous early generations (the Salaf).
Every belief that agrees with divine revelation is true, and every belief that contradicts it is false, regardless of how confident or comfortable its followers may appear.
Their sense of peace may only be outward or temporary. True happiness lies in correct faith and sincere worship of Allah.
Allah says:
"Whoever does righteous deeds, whether male or female, while being a believer, We will surely grant them a good life."
(Qur'an 16:97)
And He also says:
"But whoever turns away from My remembrance will certainly have a life of hardship."
(Qur'an 20:124)
A person may appear to live in luxury while lacking genuine inner tranquility, or this worldly ease may even be a gradual test (istidraj) from Allah. Complete happiness lies in sound faith in this life and salvation in the Hereafter.
The firmness of the people of falsehood in their beliefs, or their apparent sense of peace, is not evidence that they are upon the truth. Truth is not determined by feelings but by the revelation that Allah sent to His Messenger ﷺ. Whoever holds firmly to the Qur'an and the Sunnah and turns away from Satan's whispers will, by Allah's permission, increase in certainty and steadfastness.
And Allah knows best.