Tzaddikim Unlocking Stored Kindness

ובמדרש מביא על הפסוק האל הנאמן כו' עובדא דרפב"י ע"ש נראה דפי' שומר הברית והחסד שאוהבי' הצדיקים המה בידם לפתוח הברית והחסד בשמים כי הוא שמור וגנוז להם:

And in the Midrash he brings, on the pasuk "the faithful God" (Devarim 7:9), the story of Rabbi Pinchas ben Yair—see there. It appears that the meaning of "who guards the covenant and the kindness" is that those who love Him, the tzaddikim, have in their power to unlock the covenant and the kindness in the Heavens, for it is guarded and stored away for them.

The Sfas Emes cites the Midrash that links the verse describing Hashem as the faithful One who safeguards the covenant and kindness to a story about Rabbi Pinchas ben Yair. He explains that the very fact that Hashem 'guards' this covenant and kindness means it is kept in reserve, and the tzaddikim who love Hashem possess the ability to open and draw down that stored-away bounty from Shamayim.

Summary: The Sfas Emes draws on the Midrash, which connects the pasuk describing Hashem as 'the faithful God who guards the covenant and the kindness' to the story of Rabbi Pinchas ben Yair. He explains that 'guarding' implies that this covenant and kindness are held in reserve, stored away in the Heavens. The tzaddikim, who are the ones who love Hashem, are granted the power to unlock and draw down this hidden treasure. Thus Hashem's faithfulness lies precisely in keeping this bounty safeguarded specifically for them.