John Calvin Commentary Genesis 17:18

"And Abraham said unto God, Oh that Ishmael might live before thee!" — Genesis 17:18 (ASV)

And Abraham said to God. Abraham does not now wonder silently within himself, but pours forth his wish and prayer. His language, however, is that of a mind still perturbed and vacillating, O that (or, I wish that) Ishmael might live! For, as if he did not dare to hope for all that God promises, he fixes his mind upon the son already born; not because he would reject the promise of fresh offspring, but because he was contented with the favor already received, provided the liberality of God should not extend further.

He does not, then, reject what the Lord offers; but, while he is prepared to embrace it, the expression, O that Ishmael! yet flows from him through the weakness of his flesh. Some think that Abraham spoke this way because he was afraid for his firstborn. But there is no reason why we should suppose that Abraham was struck with any such fear that God, in giving him another son, would take away the former, or as if the latter favor should absorb that which had preceded. The answer of God, which follows shortly after, refutes this interpretation.

What I have said is more certain: namely, that Abraham prayed that the grace of God, in which he acquiesced, might be ratified and confirmed to him. Moreover, without reflection, he breaks forth into this wish when, from sheer joy, he could scarcely believe what he had heard from the mouth of God.

‘To live before Jehovah’ means to be preserved in safety under His protection, or to be blessed by Him. Abraham therefore desires of the Lord that He will preserve the life which He has given to Ishmael.