# For this cause

"Because I am an apostle"

# I also suffer these things

Paul is referring to being a prisoner.

# to keep that which I have entrusted to him

Possible meanings are 1) Paul is trusting Jesus to help him remain faithful, or 2) Paul is trusting that Jesus will ensure that people continue spreading the gospel message.

# I am convinced

"I am certain" or "I know for sure"

# that day

This refers to the day when God judges all people.