1 Corinthians 5:6-8
6 Your glorying is not good. Know ye not that a little leaven leaveneth the whole lump?
7 Purge out therefore the old leaven, that ye may be a new lump, as ye are unleavened. For even Christ our passover is sacrificed for us:
8 Therefore let us keep the feast, not with old leaven, neither with the leaven of malice and wickedness; but with the unleavened bread of sincerity and truth .
As I pointed out in the Introduction to Biblelaw101, when you see a "but" you know that the next part of the statement is going to contradict the first part. I will show you that when Paul says, "sincerity and truth" he is not saying something spiritual that we can't understand. When he says "sincerity and truth" he means the opposite of "wickedness" (lawlessness).
When Paul says to keep it in "sincerity and in truth" he using it in the same way as found in Joshua 24:14 (the only other time that phrase is said in the scriptures):
"Now therefore fear the LORD, and serve him in sincerity and in truth: and put away the gods which your fathers served on the other side of the flood, and in Egypt; and serve ye the LORD".
The Hebrew word that was translated to "sincerity" in Joshua also translated as "without blemish" and "perfect". Also, Psalm 119:142 states "thy law is truth". Both times the scriptures use the phrase "sincerity and in truth" it means, "perfect obedience to the law" or "without sin".
In Corinthians, chapter 11 Paul again tells people be without sin when he tells them to examine themselves and not to drink the Passover cup unworthily.
Before one takes the Passover meal, one must examine himself and purge out the old leaven (have repented from his sins).
1 Corinthians 11:23-32
23For I have received of the Lord that which also I delivered unto you, that the Lord Jesus the same night in which he was betrayed took bread:
24And when he had given thanks, he brake it, and said, Take, eat: this is my body, which is broken for you: this do in remembrance of me.
25After the same manner also he took the cup, when he had supped, saying, this cup is the new testament in my blood: this do ye, as oft as ye drink it, in remembrance of me.
26For as often as ye eat this bread, and drink this cup, ye do shew the Lord's death till he come.
27Wherefore whosoever shall eat this bread, and drink this cup of the Lord, unworthily, shall be guilty of the body and blood of the Lord.
28But let a man examine himself, and so let him eat of that bread, and drink of that cup.
29For he that eateth and drinketh unworthily, eateth and drinketh damnation to himself, not discerning the Lord's body.
30For this cause many are weak and sickly among you, and many sleep.
31For if we would judge ourselves, we should not be judged.
32But when we are judged, we are chastened of the Lord, that we should not be condemned with the world.