16 Circumcise therefore the foreskin [arlat] of your heart [lev], and be no more stiffnecked.

Circumcision - The token of the covenant

"This," said God, "is My covenant which ye shall keep, between Me and thee and thy seed after thee: every man-child among you shall be circumcised; and it shall be a token of the covenant betwixt Me and you, Abraham."

The appointing of this token in their flesh was the establishment of the covenant with Abraham's seed in the time of Isaac and Jacob in their generations. When, therefore, Israelites behold the mark in their flesh it reminds them that they are

"the children of the covenant which God made with their fathers, saying unto Abraham, And in thy Seed shall all the kindreds of the earth be blessed" (Acts 3:25);

that the land of Canaan, all of it, is promised to them for an everlasting possession; but that an everlasting possession in it can only be attained by belief of things promised in the covenant being counted to them for righteousness in the way of God's appointment.

... It is circumcision of the heart, of which circumcision of the flesh is but the sign of the circumcised heart of Abraham, that confers a title to the land and all its attributes. Before Israel can inherit the land for ever, and so be no more expelled by "the horns of the Gentiles," they must "circumcise the foreskin of their hearts, and be no more stiff-necked;" and "love the Lord (Jesus) their God with all their heart, and with all their soul, that they may live "

...while Moses exhorted them to circumcise the foreskin of their hearts, the Lord says that He will change their hearts Himself; not, however, by "the foolishness of preaching," for that has failed even by the mouth of apostles energized by the spirit: but by means in reserve which will astonish Israel and the world, and of which He has spoken at large in the holy Scriptures.

Elpis Israel 2.2.