Some time in March of 1525, the printer Froschauer produced Zwingli’s massive and impressive ‘Commentary on True and False Religion‘ (De vera et falsa religione commentarius). The exact date can’t be determined because neither the printer nor Zwingli mention the precise day.
The book is Zwingli’s most complete ‘systematic theology’. In it, he covers topics such as God, Man, Christianity, the Law, repentance, the Gospel, the Church, ‘Sacraments’, Marriage, Baptism, the Eucharist, Vows, Invocation of Saints, Merit, Purgatory, etc. He offers scriptural reasons for either upholding or abandoning various doctrines (as he does, for instance, on the question of clerical marriage), and suggests ways of becoming more ‘scriptural’ in outlook and practice.
Unfortunately it’s hard to find an English copy, though they are available. Fortunately the volume is freely available in its original form-
De Vera Et Falsa Religione