The sense of "if" here is "since." Our life in Christ was assured eternally when we were born again through faith in His finished work of redemption for us. When He died for us, we were "dead with Christ" (Colossians 2:20; Romans 6:8), then "buried with him" (Colossians 2:12; Romans 6:4), and "quickened together with him" (Colossians 2:13).

Source:

KJV Defenders Study Bible, by Dr. Henry Morris, Ph.D.
Publisher: Thomas Nelson
Language: English
DEF 10 ISBN 0-529-10444-x
DEF 10-1 ISBN 0-529-10445-8