Rambam on Meriting Eternal Life in the World to Come by Performing Mitzvot with Love

"It is a fundamental principle of the Torah that in order to merit eternal life in the World to Come a person must fulfill at least one mitzvah properly with complete devotion to God. This mitzvah must be performed without incorporating in it any personal interest in the observance, intending it only fulfill God will with love."

-- Rambam, Peirush HaMishnayot, Makkot 3:16

The Rambam is commenting on the following mishnah:

"Rabbi Chananaia ben of Akashya says: The Holy One, Blessed is He, wanted to confer merit upon Israel, therefore he gave them Torah and mitzvot in abundance. As it says, 'Hashem desired for the sake of Israel's righteousness, that the Torah be expanded and strengthened.' (Isaiah 42:21)." -- Mishnah, Makkot 3:16

One should clarify Rambam's conception: "... performed without incorporating in it any personal interest..." This would be negated should one be performing the mitzvah with the intention to "merit eternal life in the World to Come."
  

Rabbi Moshe Cordovero on the Letters of Emanation and Performing Mitzvot

"There is no doubt that the letters that compose each and every pericope of the Torah, and every Gemara and chapter someone is studying, which concern a certain Mizvah, have a spiritual reality which ascends and clings to the branches of a certain sefirah, namely the sefirah that hints to this Mizvah, and when the person studies the corresponding Mizvah or the chapter or the pericope or the verse, those letters will move and stir on high.

"And behold, the voices and the realities of the letters produced by the twist of the lips bestow on them a certain act and movement like that of a body. And the reality of the letters ascends and it is found everywhere on the way of their ascent from one aspect to another, following the way of the descending emanation from one stage to another."

-- Rabbi Moshe Cordovero, Or Yaqar vol. 12 p. 147 (transl. Idel in "Reification of Language in Jewish Mysticism" 1992)