In general use: To testify to, to assert, to affirm or confirm, as true or certain. Now rare.

To declare (a statement) to be true; to uphold as the truth; to corroborate, support. Obsolete.

To assert or affirm to be true or according to fact; to guarantee the truth or accuracy of (a statement, etc.); to attest or certify. Also const. ag…

with clause as object or with object and complement. Obsolete.