The fact is certain because it is impossible.

If the Tiber rises too high, or the Nile too low, the remedy is always feeding Christians to the lions.

He who flees will fight again.

All the Scriptures give clear proof of the Trinity, and it is from these that our principle is deduced...the distinction of the Trinity is quite clearly displayed.

Against Him those women sin who torment their skin with potions, stain their cheeks with rouge and extend the line of their eyes with black coloring. Doubtless they are dissatisfied with God's plastic skill. In their own persons they convict and censure the Artificer of all things.

Keep always in mind the rule of faith which I profess and by which I bear witness that the Father and the Son and the Spirit are inseparable from each other, and then you will understand what is meant by it. Observe now that I say the Father is other [distinct], the Son is other, and the Spirit is other. This statement is wrongly understood by every uneducated or perversely disposed individual, as if it meant diversity and implied by that diversity a separation of Father, Son, and Holy Spirit

It is certain because it is possible.