you sure can.. at least that's what i know about Catholic
Churches...
just walk in for the Mass and sit there.. just don't go up for
communion until you are baptised in the Catholic Church..
or you can join in the Novena Services which each church will
have a schedule for...
you can go to this website for the Catholic Churches in
Singapore => veritas.org.sg
for the various churches, you can click on the little map on the
screen to find the church which is nearest to you...
