What is the spirit of error?

"Beloved, believe not every spirit, but try the spirits whether they are of God: because many false prophets are gone out into the world. Hereby know ye the Spirit of God: Every spirit that confesseth that Jesus Christ is come in the flesh is of God:  And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world. Ye are of God, little children, and have overcome them: because greater is he that is in you, than he that is in the world. They are of the world: therefore speak they of the world, and the world heareth them. We are of God: he that knoweth God heareth us; he that is not of God heareth not us. Hereby know we the spirit of truth, and the spirit of error." (1 John 4:1-6 KJV) 

What does it mean to confess Jesus Christ has come in the flesh?

All we have to do is simply acknowledge Jesus is our Master and savior and we are saved; we don't have to do anything else?

"And this is love, that we walk after his commandments. This is the commandment, That, as ye have heard from the beginning, ye should walk in it. For many deceivers are entered into the world, who confess not that Jesus Christ is come in the flesh. This is a deceiver and an antichrist. Look to ourselves, that we lose not those things which we have wrought, but that we receive a full reward. Whosoever transgresseth, and abideth not in the doctrine of Christ, hath not God. He that abideth in the doctrine of Christ, he hath both the Father and the Son. If there come any unto you, and bring not this doctrine, receive him not into your house, neither bid him God speed: For he that biddeth him God speed is partaker of his evil deeds. (2 John 1:6-11 KJV)