यस्त्वचि तिष्ठँ स्त्वचोऽन्तरो यं त्वङ्-न वेद यस्य त्वक्शरीरं यस्त्वचमन्तरोयमयत्येष त आत्मान्तर्याम्यमृतः ॥ २१ ॥
In plain words
The one who lives inside your skin, yet is nothing the skin can claim as its own, whom the skin itself never knows, whose body the skin is, who governs it from within -- that is your self, the ruler seated inside everything, and death does not touch him.
Close to the Sanskrit
He who, dwelling in the skin, is other than the skin, whom the skin does not know, whose body the skin is, who controls the skin from within -- he is your self, the inner controller, immortal.