The Essence
While Al-Qawiyy refers to raw strength, Al-Mateen signifies extreme firmness and unshakeable steadfastness. His strength never falters, weakens, or tires, no matter how much it is exercised.
In the Quran
إِنَّ اللَّهَ هُوَ الرَّزَّاقُ ذُو الْقُوَّةِ الْمَتِينُ
"Indeed, it is Allah who is the [continual] Provider, the firm possessor of strength, the Steadfast (Al-Mateen). (Surah Ad-Dhariyat 51:58)"
In Your Duas
يَا مَتِينُ ثَبِّتْ قَلْبِي عَلَى دِينِكَ
"O Mateen (Firm One), make my heart firm and unshakeable upon Your religion."
— General Supplication (Quran 7:180)