Do the primary, secondary and tertiary keyword need words in common?

Hi guys

So on doing keyword research on GKP and keyword revealer I've found a few good keywords to target.

However is it important that the primary, secondary and tertiary keywords have words or synonyms of words in common..

i.e primary keyword is "Religious Studies"
secondary keyword is "Christianity"
tertiary keyword is "Churches"

Would it be ok to use those keywords or would it be better to use "Christianity religion" and "Religious Churches" as the secondary and tertiary keywords so as all of the keywords have a word in common?

You should be fine without having to add "religion". If you look at that keyword group, they all make sense, they are all related, and google will know this.

Thanks Just needed to know whether Google had common sense in regards to this.