On Those Who Accepted the Florentine Union
"These people admit with the Latins that the Holy Spirit proceeds and derives His existence from the Son. Yet, with us, they say the Spirit proceeds from the Father. The Latins imagine that this addition to the Creed is lawful and just, but we will not so much as pronounce it. They state that unleavened bread is the body of Christ, but we dare not communicate it. Is this not sufficient to exhibit that they came to the Latin council not to investigate the truth, which they once possessed and then betrayed, but simply to earn some gold and attain a false union? Behold, they read two Creeds as they did before. They perform two different liturgies - one on leavened and the other on unleavened bread. They perform two baptisms - one by triple immersion and the other by aspersion; one with Holy Chrism and the other without it. All our Orthodox customs are different from those of the Latins, including our fasts, Church rites, icons, and many other things. What sort of union is this then, when it has no external sign? How could they come together, each retaining his own?"
On Communion With the Latin Church
"Flee brethren! Flee communion with the incommunicable and the commemoration of the uncommemorative. Behold, I, Mark, the sinner, tell you that whoever commemorates the Pope as an Orthodox prelate is guilty. Moreover, one who minds the dogmas of the Latins will be judged with the Latins, and will be deemed a betrayer of the Faith."
On Latin Theology
"They desire also to preserve their own...and at the same time do not follow the traditions of the Fathers."
"If the Latins have not departed from the correct Faith, then we have cut them off unjustly. However, if they have departed from the Faith, regarding the theology of the Holy Spirit, to Whom to blaspheme is the greatest of all perils, clearly, they are heretics, and we have cut them off as heretics."
On Accepting Latin Converts to Orthodoxy
"We must not sanctify one of the Latin race through the divine and most pure gifts given by priestly hands, unless that one shall first resolve to depart from Latin dogmas and customs and shall be catechized and joined to the Orthodox."
On Essence and Energies of God in the Fathers
"We must not be surprised if we do not find among the ancients any clear and defined distinction between the essence of God and His energies. If, in our time, after the solemn confirmation of this truth, the partisans of profane wisdom have created so much trouble in the Church over this question - and have accused Her of polytheism - what mischief would not have been perpetrated in earlier times against this truth by those puffed up with vain learning. This is why our theologians always insisted in the simplicity of God more than the distinctions which exist in Him. It would have been inopportune to exhibit the teaching concerning the essence and energies before those who had enough trouble admitting the distinction of hypostases. Thus, by a wise economy this sacred teaching has become clarified in the course of time, God using for this purpose the foolish attacks of heretics."
On the Pope
"For us, the Pope is as one of the Patriarchs - and only if he is Orthodox; whereas, they proclaim him Vicar of Christ, Father and Teacher of all Christians. Flee from them, O brethren, and from communion with them. 'For such are false apostles, deceitful workers, transforming themselves into apostles of Christ. And no wonder! For Satan himself transforms himself into an angel of light. Therefore, it is no great thing if even his ministers transform themselves into ministers of righteousness, whose end will be according to their works'" (2 Cor. 11:13-15).
Source: P.G. 160 and translated by Holy Apostles Convent in The Lives of the Pillars of Orthodoxy.