We must be baptized (John 3:3) into the one Body in the one Spirit (1 Cor. 12:13), partake of the one bread as one Body (1 Cor. 10:17), and profess the One Lord, one faith, one Baptism (Eph 4:4-5). Like Noah’s ark, you have to be on board to be saved–like Noah’s ark in the flood, outside of which none were saved, Baptism now saves (1 Peter 3:20-21), and since we are baptized into the one Body it has the same significance as the ark.
In addition, to have fellowship or communion with Christ in His Body, you must have fellowship with those who have fellowship with Him (1 John 1:3). We are forbidden, therefore, from schisms and dissensions and must be united in belief (1 Cor. 1:10). Heresy and schism, therefore, also exclude from salvation (Gal 5:20-21; Titus 3:10-11).
Furthermore, while Christ is the head of the Church, He has also willed that the Church be led by men (Acts 20:28). He put St. Peter in charge of His one and only flock (John 21:15-17)–therefore, to be a member of Christ’s flock is to be a member of the flock tended by those who continue to exercise Peter’s ministry.
Our sins are to be forgiven through the ministry of the Church (John 20:21-22) and the faith is to be taught by those with succession from the Apostles–those who have been sent (Rom 10:15). Going out without being sent, on the other hand, is harmful to souls (Acts 15:24).
So to sum up, we should not be members of those communities that went out without being sent from the Church Christ founded, do not have their sins forgiven through the Apostolic ministers, do not partake of the one Bread, are not of one belief with the Church they left, do not acknowledge their authorized shepherds, etc…
Salvation is only found in the Catholic Church because Christ willed our salvation to not be isolated, but rather to be achieved by incorporation into a communion that is in communion with the Trinity. God is a communion of persons, and therefore that is what we are called to as well.