All therefore that can be done between two men not subject to Civill Power, is to put one another to swear by the God he feareth: Which Swearing or OATH, is a Forme Of Speech, Added To A Promise; By Which He That Promiseth, Signifieth, That Unlesse He Performe, He Renounceth The Mercy Of His God, Or Calleth To Him For Vengeance On Himselfe.

Such was the Heathen Forme, "Let Jupiter kill me else, as I kill this Beast." So is our Forme, "I shall do thus, and thus, so help me God." And this, with the Rites and Ceremonies, which every one useth in his own Religion, that the feare of breaking faith might be the greater.
No Oath, But By God By this it appears, that an Oath taken according to any other Forme, or Rite, then his, that sweareth, is in vain; and no Oath: And there is no Swearing by any thing which the Swearer thinks not God.

For though men have sometimes used to swear by their Kings, for feare, or flattery; yet they would have it thereby understood, they attributed to them Divine honour.

And that Swearing unnecessarily by God, is but prophaning of his name: and Swearing by other things, as men do in common discourse, is not Swearing, but an impious Custome, gotten by too much vehemence of talking.
An Oath Addes Nothing To The Obligation It appears also, that the Oath addes nothing to the Obligation.

For a Covenant, if lawfull, binds in the sight of God, without the Oath, as much as with it; if unlawfull, bindeth not at all; though it be confirmed with an Oath..