Despite the fact that the house of Israel is stiffnecked (prideful, stubborn) and gainsaying (disagreeable, rebellious), God mercifully reaches out to them, both roots (ancestors) and branches (descendants) of his eternal family tree, promising that if they are not hard-hearted, they can be saved in his kingdom. Jacob calls on his people to repent and “cleave unto God as he cleaveth unto you.” “Cleave” is an interesting word, with two quite opposite meanings: it can mean to divide asunder (Zechariah 14:4; D&C 45:48), and (as here) it can also mean to hold tight (D&C 11:19; 98:11)—in this case hold tightly to God, be reconciled to him and be at one with him (on at-one-ment, see commentary at Mosiah 15:2–9).
In verses 6–9, Jacob asked a series of pointed questions. His people had sobering choices to make—whether to accept or reject God’s words, his power, and his great plan of redemption.
If we do not live to merit the mercy of Christ, then “justice cannot be denied,” and we will suffer the consequences of hell, characterized by fire, brimstone, flames, smoke, and torment. For more on the “lake of fire and brimstone,” see commentary at Mosiah 3:27.