Saying Rest In Peace to people who died as non Muslims contradicts the Quran. Contradicting the Quran is blasphemy.
As the one who dies on beliefs other than Islam will not rest peacefully
If you were not, know that without your Lord's protection, no one is able to refrain from sinning and without His help, no one has the strength to obey.
Ask your Lord to protect you from that which may render you an inhabitant of Hellfire. Aameen
If you prayed today, know that you were protected from sin.
If you refrained from backbiting or talebearing, know that you were protected from sin.
If you were able to do your other obligations, know that you were protected from sin.
Purify your heart and your heart will help you to purify your deeds. Don't purify it (by neglecting it) and it will corrupt your entire body, and thus your deeds (i.e., speech, intentions, and actions).
And others had many apparent deeds (fasting, praying, teaching, etc) but because of something in their heart, they became major sinners deserving of the torture of Allaah in Hellfire.
Some people did not have many apparent deeds and were not amongst the most knowledgeable, but because of something in their heart, Allaah gave them a high status.
Strive to cleanse your heart to gain a higher status from Allah. The heart is such an amazing creation, and through it one can reach a very high rank or become amongst the worst of all creation.
The Prophet, Sallallaahu ^alayhi wa sallam, said:
إِنَّ اللَّهَ لا يَنْظُرُ إِلَى صُوَرِكُمْ وَأَمْوَالِكُمْ بل يَنْظُرُ إِلَى قُلُوبِكُمْ وَأَعْمَالِكُمْ
Allah does not judge u based on your appearance or by the amount of wealth u have, rather He judges your heart & your deeds
DELAYING YOUR PRAYERS WITHOUT A RELIGIOUS EXCUSE, IS A MAJOR SIN.
Allaah revealed in the Qur'an, Surat al-Ma^oon, verses 4-5:
فَوَيۡلٌ۬ لِّلۡمُصَلِّينَ (٤) ٱلَّذِينَ هُمۡ عَن صَلَاتِہِمۡ سَاهُونَ (٥)
Ibnu-l-Jawziyy (died 597 AH), a great Hanbaliyy scholar said: “It is obligatory upon us to firmly believe that the Self of Allaah (which is not a body nor an image nor a shape) is not confined to place nor attributed with change or moving.”
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And the ‘Sunnah’ of the Messenger is his Sharee^ah, which means the ^Aqeedah (creed; belief) and aHkaam (rules).
This is narrated by Al-Bukhaariyy, Muslim, Al-Bayhaqiyy, and others with different terms.
It was asked, “And who are these ‘strangers’ O Messenger of Allaah?” He replied:
“الّذينَ يُصلحونَ ما أفسد النّاس من سُنَّتي بَعدي”
This means: *“Those who correct the discrepancies that people made to my Sunnah after me.”*