Scriptural truth as testified to by a clear conscience before God and man. That is where anyone would find such an a clear idea testified to.
Notice the Apostle Paul's testimony of his clear conscience. If any desire to have his assurances, they need to have his conscience.
Act 24:16 And herein do I exercise myself, to have always a conscience void of offence toward God, and toward men.
Where does the Scripture state that you are to believe that you are presently in heaven and in a seated condition and "in Christ"??? If you take this "by faith" then what scripture commands you to believe that? You cannot claim Epheisans 2:6 as such a command as that is circle reasoning.