He who prays is certain to be saved; while he who prays not is certain to be damned. All the saints were saved, and came to be saints by praying; all the accursed souls in hell were lost through neglect of prayer; if they had prayed, it is certain that they would not have been lost. And this will be one of the greatest occasions of their anguish in hell, the thought that they might have saved themselves so easily; that they had only to beg God to help them, but that now the time is past when this could avail them (from The Necessity of Prayer).
Further reading
This passage presents prayer as an essential part of the Christian life. St. Alphonsus Liguori’s Prayer: The Great Means of Salvation and Perfection provides the wider teaching behind that urgency, including guidance on perseverance, confidence and mental prayer.
Prayer: The Great Means of Salvation and Perfection by St. Alphonsus Liguori (paperback) on Amazon (paid link)