Believe is related to Religious. Religious is related to Angel. Angel is related to Host. 
Estimate is related to Price. Price is related to Required. Required is related to Duty. Duty is the opposite of Voluntary. 
Believe <--RelatedTo--> Religious <--RelatedTo--> Angel <--RelatedTo--> Host 
Estimate <--RelatedTo--> Price <--RelatedTo--> Required <--RelatedTo--> Duty <--Antonym--> Voluntary 