Here is my translation of a short word by Bishop Mefody of Campanie (+1974) on Thomas Sunday:
Thomas Sunday is a day of reflection about doubts. First of all, one should not be particularly surprised by doubts or be lead by them to spiritual confusion. If doubts tempted the Lord's disciples, if the Apostle Thomas had doubts, then why should it be surprising if they appear to us? Faced with a doubt, one need not accept it like an undoubted, obvious truth. One needs to allow for the possibility of mistakes in one's thought and in one's judgment. It is best, when such tempting thoughts arise about one or another question of faith, when you are unable immediately to agree with that which the Church says about it, it is best then to put off resolving that difficult question for a certain period of time. A year or two will pass, and that which you had doubted, which had seemed unclear and contradictory, will resolve itself.See also a lecture given in 1987 by Metropolitan Anthony of Sourozh: The Church of the Councils: The Onslaught of the Intellect and the Potential of Doubt.
Where do these doubts come from? Often simply from misunderstanding, from our ignorance; because we try to judge and resolve questions without any schooling whatsoever. This is namely the case with questions of faith. For we would simply laugh at free thinkers in other areas, in medicine or astronomy, for instance. But we often try to interpret questions of faith and theology with great self-assurance without any schooling whatsoever, or after having read a book or two.
Another source of our doubts in faith is our sinfulness, for the Lord said that "the pure in heart shall see God'; we do not have this purity, and it follows that we do not have a clear contemplation of the Lord. Sometimes it can be seen clearly how someone, having descended morally, also departs from the Lord.
It can also happen that evil just whispers bad things to us, and this sometimes happens in the holiest minutes of our spiritual life. In such cases it is best to drive away these bad thoughts, as we would drive away an annoying mosquito, arming ourselves with the Jesus Prayer: "Lord Jesus Christ, Son of God, have mercy on me the sinner."
May the Lord grant us simple and pure faith. And may the Lord give us a correct attitude towards doubts.