Santos' confidence, though, may have been misplaced as mutual recrimination now threatens to derail the deal.
The FARC-Colombia agreement brought an end to a 52-year civil war that killed least 220,000 people - 80 percent of them civilians - and forced nearly six million people from their homes.
FARC, which formed in the 1960s as a left wing rebel group, demobilised thousands of fighters and surrendered their weapons to UN monitors under the terms of the deal. In return, the majority of FARC members were granted amnesty for crimes committed during the war. Five senate seats and five representative seats were also reserved for FARC’s successor party, the Common Alternative Revolutionary Force, as the movement geared up for mainstream politics.
But the Congress-approved deal remains highly divisive among wider Colombian society. Supporters of the accord say that, while imperfect, it is the best chance for Colombia to build a unified and peaceful society following decades of violence. But opponents say FARC has not paid high enough a price for its violent past and that its victims have not received justice.
The recent high-profile arrest on drugs trafficking charges of Jesus Santrich, a former FARC member and Congress member-elect who was intimately involved in the peace process, has also fuelled FARC's ire. While Colombia's attorney general says the arrest shows that the peace deal does not automatically grant impunity for drug smuggling, FARC loyalists say the government is working hand-in-hand with the US to target the movement. Murders and kidnappings by a FARC splinter group in the Colombia-Ecuador border region have prompted revulsion in both countries and undermined continuing efforts to forge a peace deal with the National Liberation Army (ELN), another leftist armed group.