Saint Aquinas Thomas 
Summa Theologica Book I [EPUB ebook] 

Because the Master of Catholic Truth ought not only to teach the proficient, but also to instruct beginners (according to the Apostle: As Unto Little Ones in Christ, I Gave You Milk to Drink, Not Meat— 1 Cor. iii. 1, 2)—we purpose in this book to treat of whatever belongs to the Christian Religion, in such a way as may tend to the instruction of beginners. We have considered that students in this Science have not seldom been hampered by what they have found written by other authors, partly on account of the multiplication of useless questions, articles, and arguments; partly also because those things that are needful for them to know are not taught according to the order of the subject-matter, but according as the plan of the book might require, or the occasion of the argument offer; partly, too, because frequent repetition brought weariness and confusion to the minds of the readers.
Endeavoring to avoid these and other like faults, we shall try, by God’s help, to set forth whatever is included in this Sacred Science as briefly and clearly as the matter itself may allow.