But when we pray to God, is it a venture in which we do not know whether we will succeed or not Or ought we to be certain that our praying will be heard?

The ground of our prayers should always be, that they will received by God, and that we shall obtain what we request as far is it is expedient for us. And therefore St. Paul says that true prayer comes from faith (Rom 10:14). For if we have no reliance upon the goodness of God, it will be impossible for us to call upon Him in truth.