Assumption of Mary (Salvific issue) Catholic Problem description
A protestant made the following two arguments about the assumption of Mary:

A catholic that doesn't accept the assumption of Mary will be excommunicated from the Catholic church
A catholic must believe in the assumption of Mary


Questions

Are these arguments correct?
Is the assumption of Mary a salvific issue for Catholics?
If it's not a salvific issue why is it &quot;good&quot; to believe in it as a Catholic?

The last question was asked from a Catholic perspective.
Public Denial of the Assumption Incurs Excommunication
Pope Pius XII defined the dogma of the Assumption of Mary, body and soul, into heaven in Munificentissimus Deus (1950):


For which reason, after we have poured forth prayers of supplication again and again to God, and have invoked the light of the Spirit of Truth, for the glory of Almighty God who has lavished his special affection upon the Virgin Mary, for the honor of her Son, the immortal King of the Ages and the Victor over sin and death, for the increase of the glory of that same august Mother, and for the joy and exultation of the entire Church; by the authority of our Lord Jesus Christ, of the Blessed Apostles Peter and Paul, and by our own authority, we pronounce, declare, and define it to be a divinely revealed dogma: that the Immaculate Mother of God, the ever Virgin Mary, having completed the course of her earthly life, was assumed body and soul into heavenly glory.

Hence if anyone, which God forbid, should dare willfully to deny or to call into doubt that which we have defined, let him know that he has fallen away completely from the divine and Catholic Faith.



Dogmas of &quot;divine and Catholic Faith&quot; (also called: de fide, de fide Catholica, or de fide divina et Catholica) are those truths &quot;proposed by the Church as revealed by God.&quot;¹
¶45 of Munificentissimus Deus makes it clear that those why deny the Assumption incur the highest theological censure: heresy. This is a &quot;Mortal sin committed directly against the virtue of faith, and, if the heresy is outwardly professed, excommunication is automatically incurred and membership of the Church forfeited.&quot;¹
1. On the Value of Theological Notes and the Criteria for Discerning Them by Fr. Sixtus Cartechini, S.J. (Rome, 1951).
The Assumption &amp; Salvation
Mary is properly called Co-Redemptrix because without her fiat (&quot;be it done&quot;; Lk. 1:38), Christ would not have incarnated; and without Christ, there would be no redemption or salvation.
Fr. Hardon defines &quot;Co-Redemptrix&quot; in his Catholic Dictionary:

A title of the Blessed Virgin as co-operator with Christ in the work of human redemption. It may be considered an aspect of Mary's mediation in not only consenting to become the Mother of God but in freely consenting in his labors, sufferings, and death for the salvation of the human race. As Co-Redemptrix, she is in no sense equal to Christ in his redemptive activity, since she herself required redemption and in fact was redeemed by her Son. He alone merited man's salvation. Mary effectively interceded to obtain subjective application of Christ's merits to those whom the Savior had objectively redeemed.

Thus, there is a very close connection between salvation and the Blessed Virgin Mother.
For more information, see at least Part II (esp. its ch. 1, 2, &amp; 3) of:

Mother of The Savior and Our Interior Life (1948) by Fr. Réginald Garrigou-Lagrange, O.P.