Pro
#1
Merriam Webster Dictionary defines Christian as "one who professes belief in the teachings of Jesus Christ."
How could you go back and forth with belief and unbelief? Even worse, how could you decide which days you do it (like every other day)? Belief is not something that can be on a schedule.
We all know that con worships Satan, but Jesus condemned this. The devil told Jesus "All these will I give thee, if falling down thou wilt adore me." But Jesus said, "Begone, Satan: for it is written, The Lord thy God shalt thou adore, and him only shalt thou serve." (Matthew 4:9-11) The Bibles says we must adore God and Him alone. This would forbid not adoring Him and also adoring others. To be a part-time monotheist, one would disobey one or both of these on the other days. We also have to adore God in the right way. Just deciding to worship God in a Christian way on one day but deciding to worship God in another way that is still monotheistic would also be rejecting what the Bible actually says.
Matthew 6:24 and Luke 16:13 say that a man cannot serve two masters. 1 Corinthians 10:21 says "You cannot drink the chalice of the Lord, and the chalice of devils: you cannot be partakers of the table of the Lord, and of the table of devils." The Bible says that being part-time will divide a man. It's impossible. The Bible doesn't say you just shouldn't but that you can't.
The Bible doesn't let you only choose to sometimes do what it says. If someone "becomes" a "Christian" but only decides to do it when he feels like it, he is not even a real Christian. "Part-time" should really just be "fake." It's just a disguise to cover up your sins and also do them while you feel like it.