Full Idea
In axiomatizing, we are merely sorting out among the truths of a science those which will play a special role, namely, serve as axioms from which we derive the others. The sentences are already true in a non-conventional or ordinary sense.
Gist of Idea
Axiomatization simply picks from among the true sentences a few to play a special role
Source
Alex Orenstein (W.V. Quine [2002], Ch.5)
Book Reference
Orenstein,Alex: 'W.V. Quine' [Princeton 2002], p.109
A Reaction
If you were starting from scratch, as Euclidean geometers may have felt they were doing, you might want to decide which are the simplest truths. Axiomatizing an established system is a more advanced activity.