Matthew Poole and continuators on Galatians 1:20

20 Now the things which I write unto you, behold, before God I lie not .

Whether those words [before God] make this sentence an Oath, is not material to determine, they are either an Oath, or a very serious Asseveration. If the Apostle designed to call God for a witness, to the correspondence of his words, with the truth of the things he had spoken, they make up an assertory Oath, which was lawful enough (though privately taken) in so serious a matter as this, where the Apostle is vindicating his Apostleship; from some Acts of which probably he had no Witnesses at hand to produce; but they may be understood (by the supplement of, I speak, or I say this) only as a form of serious assertion, to confirm the truth of what he asserted. He minds them, that he was sensible of Gods presence in all places, and particular taking notice of the things spoken; as being spoken before him, who knew that what he spake was truth.

Marginal references: Rom. 9.1.