Will Smith officially denies it, but he fawns praise on the church at every turn. Many suspect he's a Scientologist. I imagine if you have a great deal of wealth and fame, being a Scientologist gives you even more power. The Church values prominent inductees and affords them hefty privileges. An actor worth hundreds of millions can suddenly become a big influence on a multi-billion dollar religious organization with thousands of wealthy followers. Also, you can filter your taxable income and make it tax free by 'donating' to the church which is not subject to taxation in the US.
I'd join for the tax loopholes alone.
On the other hand, this basically makes his son Jaden persona non grata, as the Church has a strict anti-homosexuality doctrine. Expect much family drama if will smith is in it deep enough.