This term is cast in the future tense. As his readers are experiencing ridicule and malignment, Peter wants them to know that in the future those doing the maligning will need to “give account.” That is to say, they will need to justify why they ridiculed the Christians. The reference is to the great judgment of the Last Day (Matthew 12:36; Romans 14:10–12).
Please note that in 1 Peter 3:15 the Christian is expected to (be ready to) provide a defence to anyone who asks for the hope they cherish. Now it is their tormentors who need to explain their misconduct.
5 but they will give account to him who is ready to judge the living and the dead.