Sforno on Deuteronomy 29:15/ספורנו על דברים כ״ט:ט״ו

1כי אתם ידעתם את אשר ישבנו, and this is why you have to be concerned that there should not be among you איש או אשה, neither a man nor a woman who would seduce you to follow the vain pursuits of the nations with whom you will be living in close proximity, and who might be the cause that you would default on the undertakings in this covenant. This is the reason I have insisted that all of you be present at this ceremony, seeing that G’d examines hearts He will know who only pays lip service to this covenant and who is sincere about it. You will not be able to deceive Him.