Then Joshua built an altar unto the LORD God of Israel in mount Ebal,

Wesley's Notes

8:30 Then - Namely, after the taking of Ai. For they were obliged to do this, when they were brought over Jordan into the land of Canaan, Deu 11:29 27:2,3, which 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 under great consternation, so that all the Israelites might securely march thither. And indeed this work was fit to be done as soon as might be, that thereby they might renew their covenant with God, by whose help alone they could expect success in their great and difficult enterprize. Built an altar - Namely, for the offering of sacrifices, as appears from the following verse . Mount Ebal - God's altar was to be but in one place, Deut 12:13,14, and this place was appointed to he mount Ebal, Deut 27:4,5, which also seems 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 peace, and reconciliation with God, for the removing of the curses, and the procuring of God's blessing to sinners.

Jamieson-Fausset-Brown Bible Commentary

Jos 8:30, 31. Joshua Builds an Altar.

30, 31. Then Joshua built an altar unto the Lord God of Israel in mount Ebal-(See on [184]De 27:11). This spot was little short of twenty miles from Ai. The march through a hostile country and the unmolested performance of the religious ceremonial observed at this mountain, would be greatly facilitated, through the blessing of God, by the disastrous fall of Ai. The solemn duty was to be attended to at the first convenient opportunity after the entrance into Canaan (De 27:2); and with this in view Joshua seems to have conducted the people through the mountainous region that intervened though no details of the journey have been recorded. Ebal was on the north, opposite to Gerizim, which was on the south side of the town Sichem (Nablous).

Matthew Henry's Concise Commentary

8:30-35 As soon as Joshua got to the mountains Ebal and Gerizim, without delay, and without caring for the unsettled state of Israel, or their enemies, he confirmed the covenant of the Lord with his people, as appointed, De 11; 27. We must not think to defer covenanting with God till we are settled in the world; nor must any business put us from minding and pursuing the one thing needful. The way to prosper is to begin with God, Mt 6:33. They built an altar, and offered sacrifice to God, in token of their dedicating themselves to God, as living sacrifices to his honour, in and by a Mediator. By Christ's sacrifice of himself for us, we have peace with God. It is a great mercy to any people to have the law of God in writing, and it is fit that the written law should be in a known tongue, that it may be seen and read of all men.