One-we can tell who is in the Mystical Body of Christ. The Catholic Church is the Mystical Body of Christ and all baptized members in good standing, that is, not excommunicated or living in repeated states of deadly mortal sin, are members. This is an important point and goes back to all the blogs on baptism. If one removes one's self from the Catholic Church, such as in having an abortion, the removal must be corrected by both repentance and the lifting of the excommunication. Here I quote from Mystici Corporis Christi to clarify this idea. Not all sin destroys this link, but those listed do.
Nor must one imagine that the Body of the Church, just because it bears the name of Christ, is made up during the days of its earthly pilgrimage only of members conspicuous for their holiness, or that it consists only of those whom God has predestined to eternal happiness. It is owing to the Savior's infinite mercy that place is allowed in His Mystical Body here below for those whom, of old, He did not exclude from the banquet. For not every sin, however grave it may be, is such as of its own nature to sever a man from the Body of the Church, as does schism or heresy or apostasy. Men may lose charity and divine grace through sin, thus becoming incapable of supernatural merit, and yet not be deprived of all life if they hold fast to faith and Christian hope, and if, illumined from above, they are spurred on by the interior promptings of the Holy Spirit to salutary fear and are moved to prayer and penance for their sins.
24. Let every one then abhor sin, which defiles the mystical members of our Redeemer; but if anyone unhappily falls and his obstinacy has not made him unworthy of communion with the faithful, let him be received with great love, and let eager charity see in him a weak member of Jesus Christ. For, as the Bishop of Hippo remarks, it is better "to be cured within the Church's community than to be cut off from its body as incurable members." "As long as a member still forms part of the body there is no reason to despair of its cure; once it has been cut off, it can be neither cured nor healed." 
Two-the Church of Christ's inauguration IS the Catholic Church and not others. The only other true ones are listed in Dominus Iesus, which is linked on the side bar and here. The Catholic Church is the one, true, holy and apostolic Church. Also, I quote Mystici Corporis Christi.
They, therefore, walk in the path of dangerous error who believe that they can accept Christ as the Head of the Church, while not adhering loyally to His Vicar on earth. They have taken away the visible head, broken the visible bonds of unity and left the Mystical Body of the Redeemer so obscured and so maimed, that those who are seeking the haven of eternal salvation can neither see it nor find it.
Three-only baptized persons are children of God. See the CCC and other posts.
Four-to state that all Christian religions are the same as the Catholic Church is the heresy of eirenism, also written about many times here. We cannot pretend there are not differences and heresies. All the Protestants are heretics, and therefore, not members of the one, true Church. We invite all to enter in, but not all take up the invitation. We cannot judge, but can objectively state the situation.
Five-many priests are preaching in error about the "invisible" Church. There is no such thing. The Church is very visible and it is the Catholic Church. Here is Pope Pius XII from the encyclical again:
But we must not think that He rules only in a hidden  or extraordinary manner. On the contrary, our Redeemer also governs His Mystical Body in a visible and normal way through His Vicar on earth. You know, Venerable Brethren, that after He had ruled the "little flock"  Himself during His mortal pilgrimage, Christ our Lord, when about to leave this world and return to the Father, entrusted to the Chief of the Apostles the visible government of the entire community He had founded. Since He was all wise He could not leave the body of the Church He had founded as a human society without a visible head. Nor against this may one argue that the primacy of jurisdiction established in the Church gives such a Mystical Body two heads. For Peter in view of his primacy is only Christ's Vicar; so that there is only one chief Head of this Body, namely Christ, who never ceases Himself to guide the Church invisibly, though at the same time He rules it visibly, through him who is His representative on earth. After His glorious Ascension into Heaven this Church rested not on Him alone, but on Peter, too, its visible foundation stone. That Christ and His Vicar constitute one only Head is the solemn teaching of Our predecessor of immortal memory Boniface VIII in the Apostolic Letter Unam Sanctam;  and his successors have never ceased to repeat the same.
Six-there are many false religions. Sorry, a man-made religion is false. Again, look at the above document of the Church highlighted. Here is Dominus Iesus:
The Church's constant missionary proclamation is endangered today by relativistic theories which seek to justify religious pluralism, not only de facto but also de iure (or in principle). As a consequence, it is held that certain truths have been superseded; for example, the definitive and complete character of the revelation of Jesus Christ, the nature of Christian faith as compared with that of belief in other religions, the inspired nature of the books of Sacred Scripture, the personal unity between the Eternal Word and Jesus of Nazareth, the unity of the economy of the Incarnate Word and the Holy Spirit, the unicity and salvific universality of the mystery of Jesus Christ, the universal salvific mediation of the Church, the inseparability — while recognizing the distinction — of the kingdom of God, the kingdom of Christ, and the Church, and the subsistence of the one Church of Christ in the Catholic Church.
The roots of these problems are to be found in certain presuppositions of both a philosophical and theological nature, which hinder the understanding and acceptance of the revealed truth. Some of these can be mentioned: the conviction of the elusiveness and inexpressibility of divine truth, even by Christian revelation; relativistic attitudes toward truth itself, according to which what is true for some would not be true for others; the radical opposition posited between the logical mentality of the West and the symbolic mentality of the East; the subjectivism which, by regarding reason as the only source of knowledge, becomes incapable of raising its “gaze to the heights, not daring to rise to the truth of being”;8 the difficulty in understanding and accepting the presence of definitive and eschatological events in history; the metaphysical emptying of the historical incarnation of the Eternal Logos, reduced to a mere appearing of God in history; the eclecticism of those who, in theological research, uncritically absorb ideas from a variety of philosophical and theological contexts without regard for consistency, systematic connection, or compatibility with Christian truth; finally, the tendency to read and to interpret Sacred Scripture outside the Tradition and Magisterium of the Church.
Seven-we can tell who is in the Mystical Body of Christ. We can pray for those who are not and we should also do penance. A person who follows Peter, that is the Pope, is in the Mystical Body of Christ. Here is Pope Pius XII again on this subject:
The Church which He founded by His Blood, He strengthened on the Day of Pentecost by a special power, given from heaven. For, having solemnly installed in his exalted office him whom He had already nominated as His Vicar, He had ascended into Heaven; and sitting now at the right hand of the Father He wished to make known and proclaim His Spouse through the visible coming of the Holy Spirit with the sound of a mighty wind and tongues of fire. For just as He Himself when He began to preach was made known by His Eternal Father through the Holy Spirit descending and remaining on Him in the form of a dove,  so likewise, as the Apostles were about to enter upon their ministry of preaching, Christ our Lord sent the Holy Spirit down from Heaven, to touch them with tongues of fire and to point out, as by the finger of God, the supernatural mission and office of the Church.
34. That this Mystical Body which is the Church should be called Christ's is proved in the second place from the fact that He must be universally acknowledged as its actual Head. "He," as St. Paul says, "is the Head of the Body, the Church."  He is the Head from whom the whole body perfectly organized, "groweth and maketh increase unto the edifying of itself." 
There is a lot more. I suggest taking time and reading both documents.