Why is it important for Muslims to tell the truth?

Truth form the main infrastructure of Islam, as it is sourced from the real God, the holy book (Koran) introduces Islam the standard reference for whom seeking truth for all mankind in continuation for preceding true religions (but supersede them all),

If you truly study Islam, Koran and the life of prophet Muhammad peace be upon him and his true followers, and even the people who newly embrace Islam (in general monitor the positive changes in their life style) you will sense this meaning,