Why The Children of Reuben Built An Altar

Joshua 22:21-29
 
21  Then the children of Reuben and the children of Gad and the half tribe of Manasseh answered, and said unto the heads of the thousands of Israel, 
22  The LORD God of gods, the LORD God of gods, he knoweth, and Israel he shall know; if it be in rebellion, or if in transgression against the LORD, (save us not this day,) 
23  That we have built us an altar to turn from following the LORD, or if to offer thereon burnt offering or meat offering, or if to offer peace offerings thereon, let the LORD himself require it; 
24  And if we have not rather done it for fear of this thing, saying, In time to come your children might speak unto our children, saying, What have ye to do with the LORD God of Israel? 
25  For the LORD hath made Jordan a border between us and you, ye children of Reuben and children of Gad; ye have no part in the LORD: so shall your children make our children cease from fearing the LORD. 
26  Therefore we said, Let us now prepare to build us an altar, not for burnt offering, nor for sacrifice: 
27  But that it may be a witness between us, and you, and our generations after us, that we might do the service of the LORD before him with our burnt offerings, and with our sacrifices, and with our peace offerings; that your children may not say to our children in time to come, Ye have no part in the LORD. 
Jos 22:time to come, that we may say again, Behold the pattern of the altar of the LORD, which our fathers made, not for burnt offerings, nor for sacrifices; but it is a witness between us and you. 
29  God forbid that we should rebel against the LORD, and turn this day from following the LORD, to build an altar for burnt offerings, for meat offerings, or for sacrifices, beside the altar of the LORD our God that is before his tabernacle.