My strongest belief is reason. I believe in reasonable faith. I believe in the scientific method; I believe that the possibility of life on other planets exists by sheer mathematical probabilities.
I do not believe that space travel is possible based upon physics and the sheer distances involved.
Do we have the ability to travel within our solar system? Yes. We've reached the moon. We've placed machines on Mars and working on putting either man or woman or both there.
Do I think we'll be able to travel to our closest star Alpha Centauri, no. Four light years is way too far at the speed we are capable. A ship would take four years just to get there at the speed of light. 186,000 miles per second. At that speed a speck of dust would destroy it. Space is a vacuum, but it is not empty.
Science fiction uses devices, like warp speed or gate or cryogenics, to overcome the suspension of disbelief to further the story, and the reader understands this and enjoys the story. For an alien to travel the distances to come to earth in my reasonable estimation is not probable. Anything is possible.