Rabbi Eliyahu Lopian zt’l:

The verse says, “And let them make Me a sanctuary, that I may dwell within them” (Shemos/Exodus 25:8), upon which our rabbis comment that since it does not state “within it”, but “within them”, it means within the heart of every single Israelite.  Thus the heart is the site wherein dwells the sacred divine presence, i.e., one must feel Godliness in the heart; the source of faith is the heart, i.e., one must feel faith in one’s heart. (From the sefer Lev Eliyahu)