heart and soul

nouns

the choicest or most essential or most vital part of some idea or experience

"the gist of the prosecutor's argument"; "the heart and soul of the Republican Party"; "the nub of the story"

center, centre, core, essence, gist, heart, heart and soul, inwardness, kernel, marrow, meat, nitty-gritty, nub, pith, substance, sum

with complete faith

"she was with him heart and soul"