How can you justify the claim by that Catholic Church that marriage is indeed one of the seven sacraments instituted by Christ to confer grace?

Answer 1

The Catholic Church doesn't need to "justify" its claims. The Catholic Church is the only Church that was founded by Jesus Christ and He appointed St. Peter (and his successors to lead it until the end of the world (see St. Matthew 16:17-19). He also sent the Holy Spirit to "guide it always" even to the end of the world. He guaranteed the Church and it is only the Catholic Church which is His appointed spokesman, and He is its head (see Acts 9:1-5, and 1 Cor 12:12-14). The Church has always conferred the sacraments, including marriage, since the very beginning, long before she wrote the New Testament, which she didn't approve and canonize as we have it now until the end of the fourth century by Pope Damasus and the Council of Rome.

Answer 2

Because He did, and He gave His Church the duty or administer it, and the Church has followed His Command for the past two thousand years without ever wavering. Not only did Jesus Christ guarantee that the Church would never err, but He also sent the Holy Spirit to guide it until the end of the world, and placed His Vicar over it to rule it:

St. Matthew 16:17-19:

And Jesus answering, said to him [Peter]: Blessed art thou, Simon Bar-Jona: because flesh and blood hath not revealed it to thee, but my Father who is in heaven. And I say to thee: That thou art Peter; and upon this rock I will build my church, and the gates of hell shall not prevail against it. And I will give to thee the keys of the kingdom of heaven. And whatsoever thou shalt bind upon earth, it shall be bound also in heaven: and whatsoever thou shalt loose on earth, it shall be loosed also in heaven.