If Mary was sinless, doesn’t that make her like another god? I thought only God was sinless.
On the surface it would appear that way. But, we have to keep in mind that Mary is sinless for a different reason than God is sinless.
The three Persons of the Trinity are sinless — and Jesus, being one of Them, is sinless — by their very nature, because of the type of beings that they are. Scripture tells us that God is all good. Goodness does not only describe Him, it’s what He is. God is good, just as God is love and is truth. A Person who is entirely goodness, at an essential level, can in no way sin, since to sin is to choose evil, which is the absence of good.
So, God is sinless because of His divine nature, because of the type of being that He is.
Mary, however, is sinless, not because of the type of being that she is but because of the special grace she received from God. If it were a necessary aspect of her nature to be sinless, then she would indeed be a type of god, or another Person within the Trinity. But, she is a human being. She has a human nature.
As we all know, there is nothing about the human nature that precludes sin. In fact, one could say that we sin because of our human nature. The inclination to sin is sort of wrapped up in who we are as human beings who are descended from Adam and Eve. The same thing goes for Mary. She was due to receive the stain of original sin, and to commit sins throughout her life as all human beings do. Were it not for the intervention of God, Mary would have been just like us in all things, including sin.
Thankfully, God did intervene. He poured a special grace upon her at the moment of her conception that protected her from original sin and from committing any sins throughout her life. In fact, it was this same grace that empowered her will to say “Yes” to God when the invitation came to be the Mother of our Lord and Savior.
The important thing here is that this grace, and the sinlessness that resulted from it, was a gift. It was something that came from outside herself. Mary’s sinlessness was not the result of her nature, but instead the result of God’s own special initiative in her life. That’s the reason why Mary can be sinlessness and still be considered a human being with a real human nature just like ours.
If you think about it, Mary would have to be an honest-to-goodness human being in order to pass on a human nature to Jesus. Our Savior is both human and divine. His divinity comes from His being the Second Person of the Trinity. His humanity comes from his mother.