Was there a particular heresy that needed to be defeated that prompted this first infallible definition?
We give, and we shall continue to give, the humblest and deepest thanks to Jesus Christ, our Lord, because through his singular grace he has granted to us, unworthy though we be, to decree and offer this honor and glory and praise to his most holy Mother. All our hope do we repose in the most Blessed Virgin -- in the all fair and immaculate one who has crushed the poisonous head of the most cruel serpent and brought salvation to the world: in her who is the glory of the prophets and apostles, the honor of the martyrs, the crown and joy of all the saints; in her who is the safest refuge and the most trustworthy helper of all who are in danger; in her who, with her only-begotten Son, is the most powerful Mediatrix and Conciliatrix in the whole world; in her who is the most excellent glory, ornament, and impregnable stronghold of the holy Church; in her who has destroyed all heresies and snatched the faithful people and nations from all kinds of direst calamities; in her do we hope who has delivered us from so many threatening dangers. We have, therefore, a very certain hope and complete confidence that the most Blessed Virgin will ensure by her most powerful patronage that all difficulties be removed and all errors dissipated, so that our Holy Mother the Catholic Church may flourish daily more and more throughout all the nations and countries, and may reign "from sea to sea and from the river to the ends of the earth," and may enjoy genuine peace, tranquility and liberty. We are firm in our confidence that she will obtain pardon for the sinner, health for the sick, strength of heart for the weak, consolation for the afflicted, help for those in danger; that she will remove spiritual blindness from all who are in error, so that they may return to the path of truth and justice, and that here may be one flock and one shepherd.