"This Constitution, and the laws of the United States which shall be made in pursuance thereof; and all treaties made, or which shall be made, under the authority of the United States, shall be the supreme law of the land; and the judges in every state shall be bound thereby, anything in the Constitution or laws of any State to the contrary notwithstanding."
US Constitution, Article VI.
The writers of the Constitution didn't have the authority to abrogate the law of the land that existed before the Constitution did.
The declaration that the law of the land was to be defined by the Constitution established an ambiguity concerning the law of the land as it existed prior to that declaration and the notional law of the land as it was defined by the Constitution.
Prior to the declaration the following meaning applied:
lex terre: The law of the land. The common law, or the due course of the common law; the general law of the land. Equivalent to "due process of law". In the strictest sense, trial by oath; the privilege of making oath. (Black's 5th)
The ambiguity is apparent in the description of the oaths made by Supreme Court Justices:
Oaths of Supreme Court Justices:
"I, _________, do solemnly swear (or affirm)
Jurare est Deum in testum vocare, et est actus divini cultus. To swear is to call God to witness, and is an act of religion. 3 Co. Inst. 165. Vide 3 Bouv. Inst. n. 3180, note; 1 Benth. Rat. of Jud. Ev. 376, 371, note.
Old English að "oath, judicial swearing, solemn appeal to deity in witness of truth or a promise," from Proto-Germanic *aithaz (source also of Old Norse eiðr, Swedish ed, Old Saxon, Old Frisian eth, Middle Dutch eet, Dutch eed, German eid, Gothic aiþs "oath"), from PIE *oi-to- "an oath" (source also of Old Irish oeth "oath"). Common to Celtic and Germanic, possibly a loan-word from one to the other, but the history is obscure. In reference to careless invocations of divinity, from late 12c.
c. 1300, from Old French afermier (Modern French affirmer) "affirm, confirm; strengthen, consolidate," from Latin affirmare "to make steady, strengthen," figuratively "confirm, corroborate," from ad- "to" (see ad-) + firmare "strengthen, make firm," from firmus "strong" (see firm (adj.)). Spelling refashioned 16c. in French and English on Latin model. Related: Affirmed; affirming.