God has always demanded that our "sacrifice" to him, no matter how small, must be pure and untainted. The principal was laid down repeatedly in the Old Testament, "Do not sacrifice to the LORD [Jehovah] your God an ox or a sheep that has any defect or flaw in it, for that would be detestable to him." - Deut. 17:1, NIVSB, (Cf. Lev. 1:3,10; 3:1). Even in "grain offerings" when the grain was made into flour and baked before it was offered, there was to be absolutely no leaven or "yeast" in that offering. The leaven, then, was figurative of an impurity, comparable to a defect or flaw found in an impure animal sacrifice, and made such a flawed sacrifice detestable to God.
So, can we get away with a tiny bit of "leaven" in our "sacrifices" to God today? No more than we can get away with a tiny drop of cyanide in a glass of pure drinking water. Paul tells us of a saying that makes that very point:
"Have you never heard the saying, `A little leaven leavens all the dough'? The old leaven of corruption is working among you. Purge it out, and then you will be bread of a new baking. As Christians you are unleavened Passover bread .... So we who observe the festival must not use the old leaven, the leaven of corruption and wickedness, but only the unleavened bread which is sincerity and truth." Further explaining, Paul says: "you must have nothing to do with any so-called Christian who leads a loose life, or is grasping, or idolatrous.... You should not even eat with any such person." - 1 Cor. 5:6-8, 11, New English Bible.
God Himself said:
"And God proceeded to speak all these words, saying...You must not have any other gods against my face. Because I Jehovah your God am a God exacting EXCLUSIVE DEVOTION." (Ex. 20:1-5)
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