Joshua 8:30

Then Joshua built an altar Namely, after the taking of Ai. For they were obliged to do this when they were brought over Jordan into the land of Canaan, Deuteronomy 11:29; Deuteronomy 27:2. But this is not to be understood strictly, as if it were to be done the same day; for it is manifest they were first to be circumcised, and to eat the passover, which they did, and which was the work of some days; but as soon as they had opportunity to do it, which was now when these two great frontier cities were taken and destroyed, and thereby the coast cleared, and the bordering people were under great consternation, so that all the Israelites might securely march thither. Built an altar Namely, for the offering of sacrifices, as appears from the following verse. Mount Ebal God's altar was to be put in one place, (Deuteronomy 12:13,) and this place was appointed to be mount Ebal, Deuteronomy 27:4; which also seems to have been most proper, that in that place whence the curses of the law were denounced against sinners, there might also be the tokens and means of grace, and of peace and reconciliation with God, for the removing of the curses, and the procuring of God's blessing to sinners.