I had this discussion many times . . . . 
1Corinthians 5:8
“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.”
<<<Notice that, "let us keep the feast". Don't go trying to change the subject by asking, what is the unleavened bread of sincerety and truth. Paul said plainly to a bunch of gentiles, "let us therefore keep the feast".
If as you claim the Feasts puts one back under the Old Covenant then why does Paul tell gentiles to keep the feast, with unleavened bread nonetheless. You are all in so much error, but you stubbornly refuse to see it.>>>
Never in life and nowhere in the Scriptures told Paul anybody, <<<to keep the feast, with unleavened bread>>>. You cannot read plain English!
<<<Whenever leaven is mentioned in the Bible (22 times in the Old Testament and 17 times in the New Testament), it always (or almost always) represents sin or evil. The first instance in which this word is used is found in (Ex 12:15). Because leaven is equated with sin throughout the Bible, the "unleavened bread" pictured bread (Jesus) without sin in it.>>>
Shirley starts from an unfounded assertion; then attempts to ‘prove’ his assertion with instances of proof. His proof-text, <Ex 12:15> in no way suggests sin. In fact, it contradicts the meaning of the text because Exodus 12:15 tells the PEOPLE to remove all leaven. It doesn’t tell them to remove all their sins. What would they need the passover sacrifice for if they could have removed their sins with removing leaven from their houses?
Shirley starts from an unfounded assertion; then without any proof-case of relevance, claims he has proved “"unleavened bread" pictured bread (Jesus) without sin in it”.
The lamb for passover sacrifice had to be “without blemish” itself and typified Jesus without sin in Himself. Unleavened showed the death of the perfect lamb; leavened bread showed its LIFE.
And so with Jesus the Lamb of God who is called the “Bread of LIFE”—the sinless LEAVENED Bread of Life.
<<<a "blood sacrifice" … was not to be offered with leavened bread (Ex 23:18)(Ex 34:25)>>>
… which is wrong, opposite what is actually written.
Exodus 23:17,18,
“Ye shall bring two loaves … baken WITH LEAVEN … and ye shall offer WITH THE (LEAVENED) BREAD seven lambs… an offering unto the LORD of SWEET SAVOUR.”
Sin cannot be a sweet savour unto the LORD but leaven was.
<<<It is interesting to note, however, that for a peace offering (Lev 7:13) and wave offering (Lev 23:17) God commanded that leaven be used.>>>
Significantly, yes! Because Jesus’ LIFE brought on our peace. Again, it only confirms leaven in the Bible much rather signifies the SINLESS LIFE OF CHRIST TAKEN AWAY for an atonement for our sinful life.
<<<The key is that you only need to add a very small amount of fermented dough to make new dough rise.(Gal 5:9) describes this saying, "a little leaven leaveneth the whole lump.">>>
Yes; that’s the <key>. It’s not sin which is the key. Rather,
… the function of yeast or fermentation can be compared with the working of the Holy Spirit bringing the Light of Jesus into one’s life,
Ephesians 5:8-10 “For ye were sometimes darkness, but now are ye light in the Lord : walk as children of the light for the FRUIT OF LIGHT [SPIRIT] in all goodness and righteousness and truth proving what is acceptable unto the Lord.” A “very small amount” of Jesus’ Divine LIFE—like the yeast of life or the leaven of the Spirit—enlightens everything in all of God’s children.
There is NO justification for Shirley’s illegitimate application, 
<<<Carrying this out to sin, it can be said that "a little sin can wind up destroying the whole body.>>>
Carrying the similitude of the leaven out to the Life of the Lord is the real and positive meaning which is to assure true believers. It must be said of JESUS who is our Righteousness, that "a little leaven” of HIS LIFE will fill and let “grow with the growth of God, the whole Body of Christ’s Own." Colossians 2:19.
<<<In the second instance, Jesus compared the false teaching (sin) of the Pharisees and Sadducees with leaven (Mt 16:6-12).
In the third, Jesus warned of the leaven of Herod (he was evil and immoral: Mt 4:1-12,Mk 6:14-29, Lk 3:18-21, Lk 23:7-12) and again the Pharisees (Mk 8:15).
In the fourth, Jesus said, "Beware of the leaven of the Pharisees which is hypocrisy" (sin)(Lk 12:1).>>>
Yes indeed when it is Jesus Himself who defines the leaven of the Pharisees as hypocrisy, who will protest?!
But where it is God who Himself instructs his children on the meaning of the leaven of the Bread-of-Life, who is going to define it as the leaven of the Pharisees which is hypocrisy?!
<<<In (1 Cor 5:8), Paul also gives us a great contrast between leavened (sin) and unleavened (sin free) bread, comparing leaven with "malice and wickedness" and unleavened bread with "sincerity and truth.">>>:
Shirley gives a faulty interpretation. The <key-word>, here, is not the word ‘leaven’, but the word “OLD, leaven—the leaven-OF-MALICE and wickedness”
Why does Shirley not quote the text fully?!
Because it is an old malady of expositors to rigidly sort words into categories of no exceptions, “almost always” to bolster some or other extremist ideology.
But let’s look closer at the words which Paul used. He ends verse 8 saying, “Let us feast … with REMOVING LEAVEN / UN-LEAVENING”—which ‘leaven’?
“Let us feast in / with / by (the) removing of sincerity and truth” of the “OLD leaven” and “leaven-of-MALICE-and-wickedne ss.”
“in / with / by (the) removing”—‘en’ Preposition plus DATIVE—Modal Dative / Dative of Means / Dative and Preposition of RELATION!
So here Paul actually supplies a VIVID SPIRITUAL Christian application of the passover-feast specifically wherein is CELEBRATED THE REMOVAL OF LEAVEN rather than the addition of leaven. Here only, Paul makes the feast one of the removing of the “OLD leaven” of the PEOPLE’S “malice and wickedness” and not of Jesus’ NEW LIFE that He had obtained “in / with / by REMOVING” THE LEAVEN OF HIS SINLESS LIFE.
<<<Jesus said, "Beware of the leaven of the Pharisees which is hypocrisy" (sin)(Lk 12:1).>>>
When it is Jesus Himself who defines the leaven of the Pharisees as hypocrisy, the leaven of the Pharisees is, hypocrisy—and sin.
Hypocrisy is spiritual DEATH APPEARING to be Life.
But Jesus’ leaven IS, Righteousness and IS, LIFE—“life more abundantly!” Jesus’ leaven makes the leaven-less dough of our own lives LIVE and swell and rise and overflow our plate and cup of enjoyment in the Lord in the heat of affliction, trial and temptation.