The reason why Christians are being deceived is because they do not really trust Jesus Christ and they do not follow Him. Jesus Christ is the way, and the life and the truth and if we follow Him we will not be in darkness.
It is written that if any man needs wisdom, let him ask of God who gives to all men freely. If we ask Jesus He will guide us into all truth. He gives the Holy Spirit in those who obey Him and who love Him. The Holy Spirit is the Spirit of Truth that will guide us into all truth.
The reason why Christians are deceived is because they do not know, nor trust, nor follow Jesus Christ. Are we deceived, or are we walking in truth?
May Jesus bless you.