An Antagonist is someone who opposes, fights, or competes with another. An adversary or opponent.
In your faith walk as you launch out to believe God's word you may come across someone who consistently tries to get you to move from Faith to Fear. As you proclaim your victory they will say but what if and give you every reason why your wrong opposeing what God Word says and what you are believing.
This can be a well meaning friend, realitive or spouse. God's word is always final authority and you will have to decide to allow the Holy Spirit first place regardless of who you alienate in the process.