The bill has faced intense scrutiny for the expanded powers it gives the police and the Canadian Security Intelligence Service.
Opponents argue the bill's wording is too vague, which could lead to dangerous and unlawful measures.
Public Safety Minister Steven Blaney has said the bill is necessary to keep Canadians safe.
Now that the bill is law, what changes? Here's five differences you may notice under C-51.
1. Promoting terrorism: a jail offence
Under C-51, encouraging or promoting others to carry out terrorist acts becomes its own criminal offence under the Criminal Code. Individuals can be found guilty regardless of whether the terrorist act they are promoting is carried out. According to the bill, it could land someone up to five years in prison.
There's concern at how much this new offence could impact the right to free speech. The bill is targeted at people who encourage or promote "the commission of terrorism offences in general." Opponents argue this definition is neither defined nor clear, leading to differing interpretations and wrongful use of the law.
2. Crackdown on terrorist propaganda
Expect a larger crackdown on people who share both physical and online copies of terrorist propaganda. C-51 adds a section to the Criminal Code that allows a warrant to be issued for those who share physical copies of material a judge deems to be terrorist propoganda.
It's a more complex process if it's found online. Once a judge deems certain online content to be propaganda, they can order computer administrators to give them copies of the propaganda, take it offline and identify who posted it online. That individual can defend themselves in court, if they choose. If the judge still finds it to be terrorist propaganda, then they can ask administrators to permanently delete the content.
Again, the potentially broad definition of terrorism propoganda causes concern: "the writing, sign, visible representation or audio recording that advocates or promotes the commission of terrorism offences in general."
Could it snare more people than necessary in its web? Does it threaten free speech?
3. More arrests without warrant
Police will now have the power to preventatively arrest more people without a warrant. The bill makes slight tweaks to the current Criminal Code wording to widen the net of who police can arrest on suspicion.
Now someone can be arrested without a warrant if police believe the individualmay carry out terrorist acts. Under the previous wording, arrest without a warrant was allowed only when police believed terrorist acts will be carried out.
Police now have the power to arrest without warrant if it islikely that this will prevent a terrorist act. Before C-51, arrest without warrant was allowed if it was necessary to prevent the terrorist act.
4. More personal information shared between departments
If a person is deemed a threat to national security, government departments can now share that person's personal information with even more departments. This change falls under the "Security of Canada Information Sharing Act."
Seventeen government departments like the Canada Border Services Agency or the Canada Revenue Agency will now share information with, for example, Health Canada or the Communications Security Establishment Canada.
Is so much sharing needed? Critics say health and income tax information, as examples, don't relate to national security.
C-51 outlines a wide range of potential threats to national security: terrorism, interference with critical infrastructure, or an activity that causes serious harm to a person or his/her property because of that person's association with Canada. With such a broad range of threats, opponents are concerned about how many people could be targeted and have their information shared under these guidelines.
Does a peaceful road blockade, for example, interfere with "critical infrastructure"?
5. CSIS can now disrupt terror plots
Perhaps the biggest change is the newly-defined role of the Canadian Security Intelligence Service.
As C-51 becomes law, CSIS gets more power to disrupt suspected terrorist plots, rather than just collecting information about them. If they have reasonable grounds to think a security threat exists, CSIS can now interfere with the travel plans and bank transactions of suspected terrorists. CSIS also has the power to disrupt radical websites and Twitter accounts.
Opponents are upset with the increased power for several reasons.
CSIS was created with the mandate of gathering intelligence. Now that C-51 is law, it reaches beyond that.
CSIS also has to ability ask judges for approval in cases where their measures would breach rights or freedoms otherwise protected by the law.
Critics worry that CSIS lacks sufficient oversight to ensure these new powers are not abused.