Swearing by Saints is as forbidden as sacrificing to them
Swearing by the Saints is rejected outright.
An oath by any creature is as forbidden as a sacrifice to that creature, because Scripture condemns both swearing by what is not God and sacrificing to any but God.
A secondary oath, on the ground that divine truth shines in the Saints, has no more excuse than a secondary sacrifice on the ground that God's power and goodness appear in them.
Latin
XXVII. Similiter omnino improbant consuetudinem jurandi per Sanctos, nec minus vetitum censent per creaturas quaslibet jurare, quam illis sacrificare, quoniam Scriptura non minus illos damnat, qui jurant per eos qui Dii non sunt, quam illos qui offerunt sacrificium praeterquam Deo soli. Nec plus rationis vident ut secundario juretur per Sanctos, quatenus in iis elucet divina veritas, quam ut iis secundario quoque sacrificetur, quatenus in illis apparent insignia Omnipotentiae & Bonitatis divinae documenta.