It is commonplace in Nigeria especially practised in the Yoruba culture that, a pregnant woman should attach a needle (or any iron metal) on her dress right on the tommy whenever she goes out. It is believed that the iron metal will prevent demons from possessing or replacing the baby in the tommy. It is widely believed that demons move about especially in the marketplace therefore, pregnant women take this precaution to forestall giving birth to demon-possessed baby or loss of baby.
This simply means that, the pregnant woman believes in the efficacy of the iron metal (which is the symbol of the god of iron-Ogun) to prevent demons possessing her baby. It is a common sight to see many Christians do this in our society. Do they do this in error or out of ignorance? Should Christians who profess Christ as their saviour uphold the traditional practice of the god of iron at the same time? Should a Christian do this?

You certainly know the answer already.
You cannot serve two masters at a time Matt.6:24.
You cannot partake of the Lords table & the table of idols 1 corith 10:21.
God will not share his glory with demons.Isaiah 42:8
...what fellowship hath righteousness with unrighteousness,what communion hath light with darkness. 2 corith 6:14

^^^ Many don't see anything wrong in it. Even most highly regarded spiritual women do it. Can this single act prevent them from making heaven? Or there is nothing serious about it?

The words Jesus was said to have said:

"Give what belongs to Caesar to Caesar and what belongs to God to God."

Kay 17: The words Jesus was said to have said:

"Give what belongs to Caesar to Caesar and what belongs to God to God."

Please come clear on this