en_tn_condensed/col/01/05.md

because of the certain hope reserved for you in heaven

Here "certain hope" stands for what the believer can confidently expect, that is, the things that God has promised to do for all believers. These things are spoken of as if they were physical objects that God was keeping in heaven for the believers to possess later. Alternate translation: "because you are certain that God, who is in heaven, will do the many good things that he has promised you" (See: rc://en/ta/man/translate/figs-metonymy)

the word of truth, the gospel

Possible meanings are 1) "the message about the truth, the gospel" or 2) "the true message, the gospel."