Matthew Poole and continuators on Genesis 15:8

8 And he said, Lord GOD, whereby shall I know that I shall inherit it?

He asks a sign not out of distrust of God's promise, for he was strong in faith, Rom. 4. 20. but for further assurance and confirmation of it. And such an asking of a sign was not an unusual practise with good men, as Iud. 6. 37. 2 King. 20. 8. nor are they reproved for it: but on the contrary, Ahaz was commanded to ask a sign, and reproved for not asking it, Isa. 7.