John Calvin Commentary Genesis 13:18

"And Abram moved his tent, and came and dwelt by the oaks of Mamre, which are in Hebron, and built there an altar unto Jehovah." — Genesis 13:18 (ASV)

And Abram removed his tent. Here Moses relates that the holy man, animated by the renewed promise of God, traversed the land with great courage, as if by a look alone he could subdue it to himself. Thus we see how greatly the oracle had benefited him: not that he had heard anything from the mouth of God to which he was unaccustomed, but because he had obtained a remedy so timely and suitable for his present grief that he rose with collected energy toward heaven.

Eventually, Moses records that the holy man, having performed his circuit, returned to the oak, or valley of Mamre, to dwell there. Again, he commends his piety in raising an altar and calling upon God.

I have already frequently explained what this means, for he himself bore an altar in his heart. But seeing that the land was full of profane altars on which the Canaanites and other nations polluted the worship of God, Abram publicly professed that he worshipped the true God—and not at random, but according to the method revealed to him by the word.

Therefore, we infer that the altar mentioned was not built rashly by his hand, but was consecrated by the same word of God.