Your correspondent inquires how oaths were taken prior to their being taken on the Gospel.
Among the nations who overthrew the Roman empire, the most common mode of swearing was on the relics of the saints. In England, I think, the most common mode was to swear on the corporalia or eucharistic elements, whence we still have the common phrase "upon your corporal oath." In each case the hand was placed on the thing sworn by.
The laws of the Alamanni as to conjurators, direct that the sacrament shall be so arranged that all the conjurators shall place their hands upon the coffer (containing the relics), and that the principal party shall place his hand on all theirs, and then they are to swear on the relics. (Ll. Alam. cap. 657.)
The custom of swearing on the Gospels is repeatedly mentioned in the laws of the Lombards. (Ll. Longo. 1 tit. 21. c. 25.; Ll. Longo. 2. tit. 55. c. 2., and c. 2. tit. 34. et al.)
In the Formularies of Marculphus, two forms of oaths are given, one says that—
"In palatio nostro super capella domini Martini ubi reliqua sacramenta percurrunt debeat conjurare."
In the other we read—
"Posita manu supra sacrosanctium altare sancti ... sic juratus dixit. Juro per hunc locum sanctum et Deum altissimum et virtutis sancti ... quod," &c.
In the laws of Cnût of England, two forms of oath are given. They both begin with "By the Lord before whom this relic is holy." (Ancient Laws and Justice of England, p. 179.)
Your correspondent asks "what form of Judicial oath was first sanctioned by Christians as a body?"