I'm all in favor of people believing anything they want — except when their behavior affects others in a negative way.
To believe in any of the "big three" Abrahamic religions, you have to accept several tenets. You have to believe in supernatural miracles; that God can't make any mistakes; that God is omniscient, omnipotent and omnipresent; and that God gave us a "soul" to name a few.
Exactly when the soul enters the body — if it exists at all — is up for grabs. And if you ask any of the 36,000 Christian sects in America, you'll find different ideas on not only what a soul is, but when believers think it enters their body.
Some abide by the verse in Genesis 2:7: "God breathes life into thee," which means the soul enters at birth.
More conservative sects believe it enters at conception, never explaining what happens to the soul when the cell divides and become twins, or in the rare event when two cells combine to make one iconoclast, or getting upset when two-thirds of all "souls" (impregnated eggs) are flushed down the toilet.