John Gill Commentary Jeremiah 29:13

"And ye shall seek me, and find me, when ye shall search for me with all your heart." — Jeremiah 29:13 (ASV)

And you shall seek me, and find [me]
When persons seek the Lord aright, they always find him; a God hearing prayer; a God in Christ; bestowing favours upon them; granting them his presence; indulging them in communion with him; and favouring them with fresh supplies of his grace, and everything needful for them; every mercy, temporal and spiritual; that is, when they seek him in Christ, who is the only way to the Father, under the guidance and influence of the blessed Spirit; in the exercise of faith upon him and his promises; with fervency of spirit and ardour of mind; with diligence and importunity; with earnest desires and strong affections; and, as follows, with all sincerity of soul:

when you shall search for me with all your heart ;
which, as Calvin rightly observes, does not design perfection, but integrity and sincerity; when they draw nigh with a true heart, and call upon him in truth, and search for him with eagerness, with a hearty desire to find him, as men search for gold, and silver, and hid treasure.