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
Alphonsus speaks urgently about asking God for help and persevering in prayer. His Prayer: The Great Means of Salvation and Perfection develops that concern with teaching on confidence in God and the practice of mental prayer.
Prayer: The Great Means of Salvation and Perfection by Saint Alphonsus Liguori (paperback) on Amazon (paid link)