This question is clearly drawn from Ephesians 4:13 which says, “Till we all come in the unity of the faith, and of the knowledge of the Son of God, unto a perfect man, unto the measure of the stature of the fulness of Christ.”
First it is important to note from this verse that IT IS NOT different faiths that will get united but “we”, the people of God, who shall come into the unity of the Faith. The Faith is already ‘united’, so to speak, for “There is…one Faith” (Eph.4:4-5)! However, it is the carnal mind of man that always hinders the full expression of that Faith. It is the carnal ideas and doctrines of man that have always distorted the right perception of “the knowledge of the Son of God”. The “knowledge of the Son of God” has always been complete and perfect but ours is a world of imperfect minds trying to grasp the perfect knowledge of God. From time to time carnal ideas distort the right perception of the Truth. It is for this reason that people have conjectured different interpretations of Scripture. To correctly perceive Truth one has to be led by the Spirit!
Spiritual people die daily to their flesh and thus continually get filled with the Spirit (1 Cor.15:31). The Spirit guides them into all Truth (Joh.16:13). That Truth is the knowledge or Word of God. As we look into the mirror of that Word, we continually get transformed – “with open face beholding as in a glass the glory of the Lord, are changed into the same image from glory to glory, even as by the Spirit of the Lord” (2 Cor.3:18).
All who are on this path of dying to self, and letting the Spirit of Christ fill them, will one day grow “unto the measure of the stature of the fulness of Christ” (2 Cor.3:18). When that is accomplished, the divisions which came by carnality would be overcome because we would all be dead to every carnal idea or thought. Thus, the Unity of the Faith can only be a work of the Holy Spirit; No council of men, or a group of ministers trying to rally behind one man, can ever produce it!