I was wondering if weight lifting is sinful. In the future I wanted to gain more muscle mass and be really muscled like the people on the muscle and weight lifting channels. Also is it a sin to want to look good.
Weight lifting, like many things in this world, is not wrong in itself. Being strong has some benefits, but it can't be the focus of our lives. "For bodily exercise profits a little, but godliness is profitable for all things, having promise of the life that now is and of that which is to come" (I Timothy 4:8).
Looking good can be nice, but you have to realize that it doesn't last. "Charm is deceitful and beauty is passing" (Proverbs 31:30).
Things like this are things you do because you enjoy it or enjoy the benefits of being healthy. But if it becomes your focus in life, you've lost on what really matters.