In Parshat Tetzave we read about the special attire worn by the Kohen Gadol - two sets of clothing, one decorated with gold and the other plain white.  The Kohen Gadol was the only person permitted entry into the Holy of Holies on only one day of the year - Yom Kippur.  It would stand to reason that on such an auspicious occasion the Kohen Gadol would wear his fine clothing decorated with gold.  Surprisingly enough, the Torah commands the Kohen Gadol to wear: "he shall don a sacred linen Tunic; linen Breeches shall be upon his flesh, he shall gird himself with a linen Sash, and cover his head with a linen Turban; they are sacred vestments - he shall immerse himself in water and then don them" (Vayikra 16:4).

The Kohen Gadol as we see is commanded to wear the simpler white clothing and not the golden ones. Why? In order that "the prosecutor does not become the advocate" - the Satan would say: "yesterday they used gold to build the Golden Calf and today they seek atonement with the same gold." (see Vayikra Rabba parsha 21) The goal and task of the Kohen HaGadol on Yom Kippur was to attain atonement from Hashem for the sins of Am Yisrael - it is therefore not proper to wear gold which was used for the sin of the Golden Calf.  Till this day it is customary to wear a white kitel on yom kippur and refrain from wearing gold for the same reason.

With this in mind, we can ask why Bnei Yisrael were commanded to build a Mishkan using, among other materials, gold.  Do we not still wish to avoid the prosecutor becoming the advocate.

I found the following explanation in Talelei Orot: the Mishkan is the concentration of spirituality in a physical area - physical materials are required.  What is the basis for spirituality?  Our love of HaKadosh Baruch Hu.

Parshat Trumah begins: "speak to the Children of Israel and let them take for me donations" (Shmot 25:2).  The love, happiness to give, and generosity with which the people responded was more important than the actual contribution.  If we could build a Mishkan with bricks of Ahavat Hashem then we would achieve the ultimate closeness with the Creator.  In this world,  however, we need physical materials and we are therefore commanded to demonstrate our Ahavat Hashem by donating that which is precious to us.

We find this clearly spelled out in Shir HaShirim: "A Tabernacle for His presence has King Shlomo made of the wood of Lebanon.  Its pillars He made of silver, His resting place was gold, its suspended curtain was purple wool, its midst was bedecked with implements bespeaking love" (Shir HaShirim 3:9-10).

The pasuk states: "They shall make an Ark of acacia wood, two and a half cubits its length, a cubit and a half its width and a cubit and a half its height" *(Shmot 25:10)

If we donate gold in order to demonstrate our Ahavat Hashem, why then is the Aron Habrit made of wood?

R' Chanina said: the world was not worthy of using cedar wood, they were only created for the purpose of the Mishkan as the pasuk states: "The trees of Hashem are sated, the cedars of Lebanon that He has planted" (Tehillim 104:16), and Lebanon refers to the Beit HaMikdash, as it says: "this good mountain and the Lebanon" (Devarim 3:25).

Why was it so important that the Aron contains wood, the wood was not even seen.

Chazal explained: "a person should always be soft like a reed and not hard like a cedar" (Taanit 20b).  Cedar wood is hard and everlasting.  The Gemara there explains that when it comes to the way a person acts with his fellow man he must be soft and flexible.  However, when it comes to Torah and mitzvoth and the building of the Beit HaMikdash we must be strong and able to endure - this is the wood used for the Aron.

The Torah commands construction of the Mishkan in the plural: "they shall make a Sanctuary for Me" (Shmot 25:8).  The Torah does not belong only to the elite and the gifted.  The Torah can only blossom when Am Yisrael works together in a partnership and everyone gives their contribution towards the furtherance of Talmud Torah and mitzvoth.  May it be the will of Hashem that we work together towards creating and atmosphere of Ahavat Hashem and Ahavat Yisrael providing the strength and courage to learn and keep the Torah.  May we then merit the building of the Beit HaMikdash and the geula shleima.