It is the Church, which is "holding back" the full revelation of the Antichrist accompanied with "signs and lyingwonders" (II Thessalonians 2:6-9)
Paul encourages the Church with "our gathering together
unto Him", which must first occur before the revelation of the "man of sin" (II Thessalonians 2:1-3)
Unbelieving Israel will believe "the Lie" (Antichrist) because they did not receive "the love of the Truth (Christ)" (II Thessalonians 2:10-11 cf. John 5:43)