Abraham could not have been more serious.
Abraham was the only man in generations that had the true religion of the ancient patriarchs and had direct interaction with God. When it came time to find a wife for Isaac, it didn’t matter what she believed, her race and kinship were of the foremost importance.
The servant had to travel over 500 miles on camel to find such a girl, and she had to agree to leave her family and travel the 500+ miles back.
“And Abraham said unto his eldest servant of his house, that ruled over all that he had, Put forth, I pray you, your hand under my head, and I will make you swear before the Lord, the God of Heaven and the God of the earth, that you shall not take a wife unto my son of the daughters of the Canaanites, among whom I dwell. But you shall go unto my country, and to my kindred, and take a wife unto my son Isaac.” –Genesis
Breathings 