There is no doubt that the spirit of Christ is essential for salvation:
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. (Rom 8:9)
There is scripture to indicate that there are varying degrees of infilling beyond salvation, some for specific reasons, and I believe that one receives more as one matures in faith. More is not quite the right word, as the Holy Spirit is not a quantifiable presence, so I trust that it isn't viewed as such. I believe that Holy Spirit Himself is multi-facteted as to His attributes and manifestations, and not just a monolithic Presence.
Baptism in the Holy Spirit is referred to several times in scripture, and while there is no requirement that it be subsequent to salvation (it can be simultaneous with salvation), scripture shows that it often is. However, it should not be viewed as a reason to stratify believers. The Baptism in the Holy Spirit, I believe, is an equipping for ministry not just of the clerical or pastoral sort, but for that which God has called an individual believer to, in terms of their function in the Body of Christ. As to whether or not one has received this Baptism in the Holy Spirit, ultimately that is between the believer and God. I believe that those who seek a greater infilling of the Holy Spirit will receive such from God, and it will be more of a perceived need in the heart of that believer, than an outward requirement sought by others.