1. If there is no ultimate source of value, no meaning-conferrer, then life is meaningless.

2a. But life is not meaningless.

or

2b. We cannot act as though life were meaningless.

Therefore,

3a. There is an ultimate source of value, a meaning-conferrer.

or

3b. We act as though there were such an ultimate source.

4. Such a ultimate source of meaning is God (or that which has meaning in itself).

or

4b. We act as though there were a God.