The following is by Ulrich Zwingli, and is from the epilogue of his Commentary on True and False Religion.
A Christian, therefore, is a man who trusts in the one true and only God; who relies upon His mercy through His Son Christ, God of God; who models himself upon His example; who dies daily; who daily renounces self; who is intent upon this one thing, not to do anything that can offend his God. Such watchfulness demands so much diligence and zeal that anyone would need many a Theseus to defend his blamelessness, and yet would never come off victorious. The Christian life, then, is a battle, so sharp and full of danger that effort can nowhere be relaxed without loss; again, it is also a lasting victory, for he who fights it wins, if only he remains loyal to Christ the head. Thus has God willed that man be an amphibian among the creatures, dwelling sometimes on earth, sometimes in the heavens; and, again, while on the earth sometimes conquering, sometimes yielding; but we are by no means to ask the reasons for His acts. Since, then, God asks of us these things only, faith and blamelessness, no more baneful plague can be imagined than a varied worship of God, the invention of our own industry. This we (being given to magnifying everything of our own) embrace, instead of that true worship of God which consists of faith and blamelessness; and, according to the words of the prophet Jeremiah, 2:13, “We have committed two evils; we have forsaken God, the fountain of living waters, and hewed us out cisterns, broken cisterns, that can hold no water.”