But that presuming again, that God is the only choice. Once you make that argument you open it up to about a thousand possibilities. Each has a chance of being wrong? Would you really want to bet on God, when it could turn out to be Zeus who rules the cosmos? Nor I doubt that Anubis is going to be too impressed with such a wager...which means your heart going to get snacked.
This isn't how traditional religions functioned. The issue was not whether you believed in the existence of Anubis, let alone having faith in him, but rather the actions that you had undertaken in life and how well they had conformed to the requirements of Ma'at.
Thus, being a Christian in itself would only condemn you to Ammut's jaws if Kemetic religion was true if and only if being a Christian necessarily required actions that deviated sufficiently from Ma'at and towards Apep that would preclude your heart from conforming to Ma'at.
This may well have been the case for early Christians who refused to participate in civic religious ceremonies, but if Kemetic religion was true and also if the requirements involve a high specificity of the ceremonies we would all be swallowed up because those ceremonies have not been practiced for close to two thousand years.
The specificity almost certainly wouldn't have to be high, if you had engaged a practicioner on this and they had had a philosophical bent, as most traditional religions recognized each other as variations on a theme. And in addition, Apep hasn't swallowed the Sun and introduced endless chaos into the world, though of course Mr. In The Name Of Love would almost certainly disagree with that notion. But for analytical purposes, simply participating in social rituals and ceremonies that act to hold the human world together is probably enough, beyond the moral requirements recorded in the Book of Going Forth By Day.
This kind of mirroring of a very narrow interpretation of Christianity (certainly something which entirely ditches the parable of the sheep and the goats and the letter of James cannot be said to be truly engaging with the New Testament) is a very bad way of thinking about other religions. For a more modern example, being a Christian is not automatically relevant to Buddhism and what happens after death in it- your karma is a matter, again, of your actions.
Now, that being said, most conservative Christians do undertake actions that would accumulate karma of a negative disposition, or would deviate from the requirements of Ma'at, or would face a stern condemnation from Minos and Rhadamanthus, or would exclude them from Ásgard and Vanaheim. But those actions are still up to them and are motivated by religious doctrine only at the level of subsects, rather than at the level of sects or of the religion as a whole.