وَالإِقْرَارُ بِهِ أَيْ بِاللِّسَانِ
And professing it verbally.
The testimony of faith and other clear professions make inward belief publicly recognizable. Since people cannot directly inspect one another's hearts, profession is a principal external basis for the community's legal dealings.
Verbal profession and inward assent are not identical in every respect. A hypocrite's words may outwardly resemble those of a sincere believer, although their inner state differs. Hence the treatment of outward rulings and inward reality must remain distinct.