This is by St Alphonsus Liguori, Doctor of the Church and founder of the Redemptorist Congregation. It is from his book on prayer being a powerful means of salvation:
"But to return to the question just proposed: Are we obliged to have recourse to the intercession of the saints? I do not wish to meddle with the decision of this question; but I cannot omit the exposition of a doctrine of St. Thomas. In several places above quoted, and especially in his book of Sentences, he expressly lays it down as certain that everyone is bound to pray; because (as he asserts) in no other way can the graces necessary for salvation be obtained from God, except by prayer:. ‘Every man is bound to pray, from the fact that he is bound to procure spiritual good for himself, which can only be got from God; so it can only be obtained by asking it of God.’ Then, in another place of the same book, he proposes the exact question, ‘Whether we are bound to pray to the saints to intercede for us?’ And he answers as follows – in order to catch his real meaning, we will quote the entire passage: 'According to Dionysius, the order which God has instituted for his creatures requires that things which are remote may be brought to God by means of things which are nearer to him. Hence, as the saints in heaven are nearest of all to him, the order of his law requires that we who “remaining in the body are absent from the Lord,” should be brought to him by means of the saints; and this is effected by the divine goodness pouring forth his gifts through them. And as the path of our return to God should correspond to the path of the good things which proceed from him to us, it follows that, as the benefits of God come down to us by means of the suffrages of the saints, we ought to be brought to God by the same way, so that a second time we may receive his benefits by the mediation of the saints.
Hence it is that we make them our intercessors with God, and as it were our mediators, when we ask them to pray for us.’ Note well the words – ‘the order of God’s law requires’; and especially note the last words – ‘as the benefits of God come down to … s by means of the suffrages of the saints, in the same way we must be brought back to God so that a second time we may receive his benefits by the mediation of the saints.’ So that, according to St. Thomas, the order of the divine law requires that we mortals should be saved by means of the saints, in that we receive by their intercession the help necessary for our salvation.
He then puts the objection, that it appears superfluous to have recourse to the saints, since God – is infinitely more merciful than they, and more ready to hear us. This he answers by saying: ‘God has so ordered, not on account of any want of mercy on his part, but to keep the right order which he has universally established, of working by means of second causes. It is not for want of his mercy, but to preserve the aforesaid order in the creation.’
In conformity with this doctrine of St. Thomas, the Continuator of Tourneley and Sylvius write that although God only is to be prayed to as the Author of grace, yet we are hound to have recourse also to the intercession of the saints, so as to observe the order which God has established with regard to our salvation, which is, that the inferior should be saved by imploring the aid of the superior. ‘By the law of nature we are bound to observe the order which God has appointed; but God has appointed that the inferior should obtain salvation by imploring the assistance of his superior.’