John 16:13 "Howbeit when he, the Spirit of truth, is come, he will guide you into all truth...". Jesus was letting us know that when the Holy Ghost was given and received, He could then lead and guide His people through the Spirit. Paul told the church at Rome in Romans 8:8-9 "So then they that are in the flesh cannot please God. But ye are not in the flesh, but in the Spirit, if so be that the Spirit of God dwell in you. Now if any man have not the Spirit of Christ, he is none of his."
These verse hold so much I cannot tell a fraction within this post, but one important fact is without the Spirit of God DWELLING in us, we are not "in the Spirit" and, therefore, we are not being led by the Spirit. Which means we are being led by the flesh which means we are not pleasing to God.
Verse 9 tells us plainly, if we have not the Spirit, we are not His. So I ask you, how important do you think the Holy Ghost (aka the Spirit of God) is to your salvation?