A rule of Bible study is never to base a doctrine on the meaning of a Greek or Hebrew word, and this controversy is a prime example. It is true that artos, used in all of the gospel accounts for the bread eaten during the Last Supper, is the Greek word for "bread." However, this word is a very general or generic term, much like the English word "bread" is. We use "bread" for everything from white to whole wheat to pumpernickel bread. We also use it for breads made of corn, barley, rye, spelt, rice, and other grains. We use it for sourdough as well as for sweet breads like banana and pumpkin. And, most importantly, we use it for both leavened and unleavened breads. We even use it as a generic term for food (as in "our daily bread")! The Greeks did the same with artos.
Greek also has a word for "unleavened," azumos (also transliterated as azymos), which is literally "without yeast." Yet, just because this word does not appear in the gospel accounts of the Last Supper does not mean that the bread Jesus and His disciples ate was leavened. Gerhard Kittle's Theological Dictionary of the New Testament, a pre-eminent source on New Testament Greek, says this on the word azumos:
P. Fiebig [a Greek-language scholar] . . . shows that the term artos does not exclude azumos, but that in certain circumstances, e.g., in description of the Passover, it may mean this. Hence the occurrence of artos at the Last Supper is no proof that this was not really the Passover.
In addition, both early Jewish writers Josephus and Philo use artos in their description of the matzo of the Passover meal. Also, the loaves of the unleavened showbread in the Tabernacle and Temple, were regularly called artoi (plural of artos). It is understandable, then, that the gospel writers used the generic term artos because they knew that their readers would know what kind of bread they were talking about.
We also need to understand the "sop" itself. This is the Greek word psomion, and means "a morsel," "a crumb," "a bit," "a fragment," or as Strong's interprets, "a mouthful." Thus, it means a piece of food, and in the Last Supper, one used particularly for dipping. Therefore, the word does not necessarily suggest that the sop was used for soaking up liquid. It could also be used like a potato or tortilla chip for dipping in a sauce or for scooping up smaller bits of another food toward the mouth. We have a traditional picture in our minds of Jesus dipping a piece of bread in gravy or something akin to salsa, but John 13:26-27 does not tell us what Jesus dipped the piece of bread in. It could have been yogurt, gravy, oil, a sauce, or any number of other things. Thus, that the bread must have been leavened so as to be soft and absorbent is not contemplated in the term.
Lastly, it is nearly a certainty that the bread Jesus and His disciples used during the meal was the same bread that Jesus used to teach them the Passover symbol of the bread as representing His broken body (Matthew 26:26). His body did not contain any sin! Leaven is a primary biblical symbol of sin and corruption. Would Christ want His disciples to memorialize His sacrifice every year by thinking of Him as leavened, that is, sinful? Certainly not! We are to remember that He sacrificed Himself as the perfect, sinless Lamb of God to pay for sin in our stead (I Corinthians 5:6-7; Hebrews 9:11-14: I Peter 2:21-24). In fact, taking the Passover with leavened bread is tantamount to blasphemy, as it distorts and repudiates the sinless sacrifice of our Savior.
Because Jesus fulfilled all righteousness (Matthew 3:15)—meaning, He did everything perfectly—it is safe to conclude that the sop, and thus the bread in the Passover symbol, was unleavened.
Christ, Our Passover
Death of a Lamb
Why We Must Put Out Leaven
Discerning Christ's Broken Body
Holy Days: Unleavened Bread
Holy Days: Passover
The Five Ws of Deleavening