Because it expresses the ideal at which all must aim, the idea of perfection has always found its place in Christian thought. In the first centuries of vigorous faith strengthened in persecution there was less need for the intellectual systematization that was to come much later. Our Saviour did not present His disciples with a complete and coherent body of abstract truths. His doctrinal discourses and discussions had the practical purpose of teaching the way to salvation, and it was the first preoccupation of the Church to preach Jesus crucified. In the course of the centuries the living Church, faced with controversies and doctrinal extravagances, would find it necessary—relying on the promise of divine assistance—to define the implications of the teaching of Christ, and this in turn would make a fuller synthesis possible and even necessary. In the meantime the different obstacles to the Christian life encountered by men of different ages and cultures inevitably led them to stress different elements in the fullness of the Christian life. The Fathers of the Desert had escaped from a profoundly corrupt society, and as a consequence they emphasized the need for penance, and purity of heart, and avoidance of sin, so that the conception of perfection which can be disengaged from their lives and writings is of a more negative character; the positive side of Christian life is not absent, but it is less in relief.