During the time of separation between the marriage covenant and the coming of the bridegroom for the bride, it was possible for the bride to commit adultery by giving herself to another man. It's possible for believers to commit spiritual adultery against Christ before He returns to take His bride to His Father s house. Paul expressed his concern over this possibility when he wrote to the Christians at Corinth. James expressed the same concern in some very strong language earlier. Whosoever therefore will be a friend of the world is the enemy of God.
Spiritual adultery is committed when a believer becomes devoted to the things of the world.
I think we see as much spiritual adultery in the world today as we do physical adultery. We all need to examine our lives and our motives and direct them to the things Christ would have us to do. If we have been unfaithful to our heavenly bridegroom, we need to confess it and trust the Holy Spirit to renew our devotion to Him. Meanwhile we wait for the Rapture when our Savior will come with a shout and take us to be with Him. That can be at any moment.